don‘t work only on your chinups, a good improvement of all your physical strenght, upper body as well as lower body will help. Better abs helps for every exercises as well as running. Strenghtening triceps and biceps will help both pushups and chinups. Better then the gravitron, find a buddy to hold your legs will you do your chinups, when you need a little help you just push yourself up with your feet. The real good advice, join a gym and ask for a good endurance plan.