Ruckmarch
The point of marching with a ruck is not to build your strength or endurance - you do better targeting PT (running, callisthenics, and weights) for that. The point of training with a ruck is to teach your body how to carry a ruck. It's called CONDITIONING for a reason. You do it so that your body is conditioned to it. If you try to walk or run with a ruck using the same body mechanics as you do without a ruck, you'll get hurt. The bottom line is that you'll carry heavy rucks for a long time in the Infantry. It may be on forced road marches. It may be in the field. You may have to run with a ruck. Your ruck may weigh 60 lbs, it may weigh 150 lbs. The weight will depend on your mission, but I can guarantee you'll be running with your ruck in the field at some point regardless of how heavy it is. That's just a fact, and all the army's official "No running in combat boots/no running with a ruck on" is bullshit. Plain and simple.
Take a marathon runner and make him swim a mile in a pool. He has the cardiovascular and muscular endurance to do it, but he doesn't have the technique. He'll be winded after one length. After practice though, his body gets used to the motions and becomes more efficient at it. Pretty soon he can swim the whole mile. Same thing with humping a ruck. The reason you do it is so your body can become efficient at it. You can't learn it in a book (or on a website). You just have to go out and do it. If your knees hurt then try less weight.
Don't get me wrong, it will always suck. It's weight on your back that wasn't really meant to be there. If you enjoy it, then you are not pushing it nearly hard enough. Ruck marching sucks. That is a universal truth. The only satisfying thing about a ruckmarch is dropping kit at the end. But if you train your body to do it, it will suck less and you'll be less likely to get stress fractures or other injuries from carrying a load your body doesn't know how to carry.
Carrying it:
Get'cher ruck up high on your back and tighten those straps. Get rid of the POS ruck the Army gives you, and get a '64 pattern ruck (the so-called 'jump ruck'). Get some A7A straps. Suck everything down tight. A tight load is a light load. Use your waist belt. Get the weight down onto your hips, and off of your shoulders. All the weight should rest on your hips. The shoulder straps are just there to keep the weight in close to your body. You do not want that weight pulling away from your body, believe me. If you like, you can tighten the waist belt, loosen the shoulder straps, until the pain is too much then tighten the shoulder straps and loosen the waist belt. And change again. It doesn't actually do any good, but it tricks your mind into thinking it helps. Practice is the only key. The only way you can get in shape to hump a ruck is by humpin' a ruck. You'll get used to it. After a couple hunnerd miles you'll start missing it.
I try to put the extra weight flat against, and centred across my shoulder blades, as high in the ruck as I can get, in the valise if possible. This is a lot more comfortable than having it way down in the bottom of the ruck, pulling you backwards. I always tried to put the heaviest stuff in between the frame and the valise, such as mortar rounds, sand bags, etc. Eventually though, when you get to BN, you're going to have your ruck packed with mission-essential kit . Expect to be carrying all the extra accessories that you're responsible for, rations, ammo, water, batteries, etc. Then, on top of that, you'll have your body armour attached to the outside of your ruck, which makes sort of an experiment in Physics, when you start to contemplate gravity and how much heavier something feels when it is strapped to your body, yet is still 3 feet away from your back. (Ya know, leverage, and such.)
Now, your pace: Well, the minimum standard is a 13 km ruckmarch completed in 2 hours, 26 minutes. On regular morning PT, you don't go quite as far, but you usually go much faster. On an advance to contact, you may go a little slower, but that just means that you're carrying that sumbitchin' weight on your back for that much longer. It just means, that at every halt, you're taking a knee and getting back up with that 60, or 80, or 120 lbs on your back. When humpin' cross-country you may go slower, but now you're goin' through woods so thick, you can barely squeeze that ruck through them. You're wadin' through swamps and rivers. You're twistin' your ankle at every second step, and slippin' down slopes. And the entire time, you'd best not make a damn sound, and you'd damn sure better be keepin' your head up and watching your arcs. You'd better be passin' the count up, and signals back. It's easy on PT - put'cher head down and give 'er. It's a whole different world in the field with a Light Infantry Company.
Don't run. There is no need for you wannabe's to run with a ruck, yet. You won't do it in Basic, and it will just cause you to injure yourself, setting your training back. Ruck faster, or with someone else who is faster, till your body learns to carry the weight, and either take longer steps, or more of them.
Starting out:
Put sand bags in the ruck. Failing that, buy bags of kitty litter, top soil, or dry cement. Steer clear of free weights, especially dumbbells. Free weights and rocks hurt. While marching they'll eventually become unbalanced, shift, and the ruck march will quickly become a self-inflicted torture session. So not only are you in unbearable pain, but you also realize what a dumbass you are at the same time. It doesn't help motivation too much. For you wannabe's, stick with sandbags or the like. They'll mould themselves your back more easily, and make it easier for you to learn how to hump. No need to add extra misery by throwing in pointed items, or solid stuff that'll just dig in. Sand bags are easier to work with, too. You can get several with different weights, label them, and use them as needed.
35 pounds is fine right now for your first time rucking. Take it slow. Work your way up slowly but steadily. Your goal should be about 60 lbs. When you can hump a 60 lb ruck for a couple hours at a good pace, you're pretty well at standard. Anything Basic or Battle School throws your way will be fairly easy to cope with. Don't just ruck up and start humpin' either. Make sure you're wearing comfortable, broken-in footwear. Two pairs of socks, etc, all the little tips I passed on earlier in this thread. Now, start light, start slow, and start easy. Just a few klicks is good, say 3 or 4. Work your way up in weight and distance. For the love of Pete, don't run out and throw 100 lbs in a backpack and try to run for 5 klicks! Or even put 25 lbs in, and run. Baby steps. Start short, slow, and light, and work up to long, fast, and heavy. If you screw up, and are say, 10 klicks into your walk when you notice significant blistering, I hope you have a cell phone. You will screw up your feet for at least a week by walking back.
Form:
If it's pulling you backwards, you need to lean forward some. Maybe you need to re-arrange the weight load. You may have the heavier stuff to low, or too far out from the body. Either that, or start going with a little less weight. It's about giving yourself a workout, learning to hump, and conditioning the body, not looking cool while doing it. Trust me, there is no way to look cool when humpin' a ruck. LCF is low, almost negative. Humping a ruck sucks ass, always has, always will.
Relax the upper body completely. From your shoulders to your fingertips, relax your entire arm. Don't tense your shoulders. Let them relax. Once the shoulder straps have bitten far enough into your trapezius, you won't feel them anymore, anyway. Let your arms swing naturally, don't force them, unless you need to pick up speed. Swinging your arms makes many people faster. I have seen guys go faster when they picked up a weapon for the walk, because when they held it with both hands, they swung into the rhythm. Lean forward slightly, but not too far. Use that to generate 'momentum'. Let your feet skim the ground, and place your feet down, don't drive the heel in. Reach from the hips and let the legs carry you along.
You need to train your body how to carry a ruck. Everything from the forward lean, to the position of your head, to how your carry your weapon, to how high you lift your feet, to how long your stride is - all of that falls into place over time. Miles and miles (and miles and miles and miles). At first it will hurt because you're putting your heels down too hard, or your stride is too long, or you're tensing your shoulders more than you need to - but after many miles of rucking it becomes more natural. You break your body in. You throw a ruck on your back and your body naturally assumes a rucking position. Your pace and gait changes to what it should be with a ruck on your back. Your motions become smoother so the ruck doesn't bounce as much.
Feet:
Take care of your feet. They can make or break you. Nothing sucks more than having the desire to complete the mission and having your feet bleeding, cracked, and infected. Your feet serve to support you and your load, absorb shock, and to provide balance and forward motion. Your feet need to be tough, yet getting them there means they have to be protected, and cared for. This is achieved by rucking in well broken-in, but not broken-down boots, by monitoring your feet's status, and knowing how to care for them when problems arise.
Rotate different pairs of boots or running shoes from day to day. Make sure that the boots are comfortable, getting the proper insoles or orthotics as required. Sizing is also critical. Try on new boots with the socks you intend to wear. Get them slightly large, as most people's feet will swell a half size or so on extended walks. Socks are just as important. Wear new, clean, correctly sized socks. They will help absorb some of the punishment, and thin, ratty, old socks do little to assist in protecting your feet. (I know some of this is repeated, but it bears repeating, so listen up.) When breaking in your new boot or shoes, use them for shorter walks. They're going to hurt.
As you break in your boots, you will notice painful "hot spots" forming. Stop and treat them as soon as the pain becomes noticeable. Any hot spot areas starting to redden should be closely monitored, and moleskin applied as needed. An experienced Medic is probably the best source for advice. While rucking, you should plan on stopping for a break for five or ten minutes every hour or so. Do not waste that time sitting on your ruck. Take the ruck off, take your boots off, and examine your feet. If they look good, let them air out for a minute or so, powder them, and change socks. Once you have started to blister, you will lose training time waiting for them to heal. Do not let them get that bad.
Never ruck with wet feet. Even well-conditioned feet are vulnerable and soft after a good soaking. Do not try to combine training activities (like swimming and rucking), and carry a spare pair of boots and several pairs of socks to change into should the ones you are wearing become soaked. (Now this doesn't hold when you get Battalion. You WILL march with wet feet. Deal with it.)
Areas of your feet that get a lot of friction and contact will start to harden and calluses will form over time. This is a good thing. The dead material of the callus will absorb the friction and impact that would hurt the skin on your feet. Soon enough, your feet will become the leathery flippers that all infantrymen have. I've seen some guys hold lighters to their feet, the callus was so thick. Most people find that issue boots will cause calluses to form on the balls of the feet, the heel, and on the outside of the foot, depending on the contact points of the boots on your feet.
As you walk, the boots and your feet will develop a symbiotic relationship. The boots will soften and begin to flex where required, and the contact points on your feet will toughen up. Eventually, your boots will be almost as comfortable as a pair of slippers, and your feet will be tough as nails. You will need to keep your nails trimmed properly to prevent injury or damage. But that, and foot powder will be all the maintenance you will require. Even if you walk long distances in combat boots your feet will blister, however, and you may even lose feeling in your feet.