A few days ago I was thinking about my running and wondering why I've been so tired the past few days. I had taken rest days and had slowed down, and I still felt tired. I'm not sure why I did it, but I added up my caloric intake for that day, and I couldn't believe my eyes: 600-700 for the whole day. After I had double checked my arithmetic to be sure I hadn't missed some food, I thought to myself, "No wonder I'm so tired!" I need about 500 calories per day (average) just to make up the energy spent in running, plus approximately 2000 calories for normal body functions. That is, roughly, 2500 calories per day, and I got only 600-700.
One of the things I've noticed as I get older is that I don't feel hungry very often. I get busy with my projects and forget about eating, and I don't feel hunger-pangs to remind me to go eat a meal. All of a sudden it's bed time, and I haven't eaten much. So, I'll just think food all day long.
For the past few days, since I realized how few calories I'm eating, I've been making a positive effort to eat regular meals, and that paid off in today's run.