... your diet. Depending on what your current goals are, exercise is usually paired with dieting for the express purpose of losing weight. Losing weight too fast may not only shock your body into a sort of panicked metabolism where it slows the burning of calories for survival, but rapid weight loss otherwise fails to apply memory in your body as steady and reasonable weight loss numbers do.
A great goal, while exercising moderately (just because it's good for you) is to learn good eating practices. Start from the basics (food groups etc.) and learn what good foods can do for your body and your energy levels. We always stress whole grains; grains with all their natural parts, i.e. the bran and the germ, take longer to digest. A whole grain diet takes longer to digest, because the physical and chemical food breakdown of the complete grain occuring in your stomach and intestines requires more time. This stabilizes the energy available in your blood for alertness.
Highly processed foods give you rushes of too much energy, followed by crashes of low energy - something we can avoid by eating less processed foods, and perhaps graduating to a diet which includes more raw and live foods. Additional benefits to whole grains, include increased fiber intake, which can improves digestion and may be linked to healthier digestive tracts partly because of the physical cleaning it can do and other benefits.