A primary source of this in our diets is Carbohydrate. It has been found that if we deplete this, say going on a long fast run, then reduce our exercise and eat plenty of carbohydrate then the body will over compensate and store extra glycogen. Some runners in long events will carbo load to make sure their glycogen stores are fully topped up just before a long race. Cross Training To get better at running you really need to run. Firstly there is little real benefit if you are well trained.
Cycling can help by strengthening opposing leg muscles thereby avoiding injury.
Cycling uphill has some cross over with hill running. Weight training can also strengthen opposing leg muscles, strengthen stomach muscles and the upper body although this may or may not affect performance. Swimming can help with recovery from pavement pounding and hard sessions. Diary Running diaries are a significant potential performance enhancer. Analysis of well filled in diaries can lead to understanding of how you improve, and also show how to avoid injury.