I think also it is important to recognize that that 'low' feeling you have is prolly part mental. I know that the only way i get running time in, is the fact that I set a side an hour or so out of every day, and dispite the internal grumbling, the moment one foot gets infront of the other, the energy is back. The brain is the critical factor in most cases. As was stated before, there is an industry out there dedicated to 'quick energy', but its all bunk, and though caffeine has a noticible affect, its better to be able to do it without a crutch to depend on, because the one time that the crutch is not there, people end up making excuses to stay on the couch rather than do something.
Whats in your head controls the body, manages pain, understands the limits, and at time pushes through them. The trick is to tame that.
Just my thoughts.