I'm a college student. 5'9", 140lbs, with low body fat. Until last week, I was fairly sedentary. This week, I've been riding a bicycle to class (.75mi each way) and on the recreational trail near the campus.

I don't have any hard numbers (I'll have to bring a watch and start a log), but after about 20 minutes of cycling, I was pooped, and not very far from where I started. My impression was that my legs were tired, rather than that I was out of breath. Now I have a mild case of delayed onset muscle soreness.

I'm not in any particular hurry to get in better shape, but I still want to know how to best build endurance and speed. Specifically, how do I know how hard to push myself? Should I cycle as hard, when I can still feel the previous day's workout in my muscles? Should I attempt to mantain a constant level of exertion during a ride? Take breaks?

Also, how important is eating? I occasionally find myself hungry with no food handy (in fact, I'm in exactly that predicament now). Eating is a chore, and I don't eat unless I'm hungry.