I have tried to run and do other high intensity exercise without adjusting my diet and really struggled. What I do now is up my carbs. So if I'm about to go for a run, I drink 200ml of fruit juice before I go and maybe some during it (depends how long I'm out). If I'm hiking up hills I eat dried apricots or flapjacks made with oats, butter and Splenda. This has completely revolutionised my exercising on LC. I suffer no 'over-carbing' effects or weight gain - it just means I can exercise properly.
Some folk manage without this I know - I just couldn't.