I am now attending fitness sessions at the police academy two nights a week. They are three hours long. I'm in pretty good shape (decent strength, bomb hill sprints) but we run 3 to 4 miles and then do calisthenic circuits. After the run I was super lightheaded and crapped out during the calisthenics. It wasn't so much that it was too much for me to handle, I just had no fuel. What should I do or supplement with to prevent this problem in the future?