Everything's been said already, but something I found is that even though I do plenty of sports during the week already, commuting by bike for a few days in a row does drastically increase the amount of food I need to take in. I find it nearly impossible to get enough calories from ordinary meals to make up the commuting deficit. I've resorted to buying big boxes of mixed nuts & raisins to snack on, and those do the trick nicely. Right now it is 1:44 PM and I'm feeling hungry again even though I ate plenty at noon. Stomach getting empty and my body is saying "it's not enough".
Even if you're overweight right now, it might pay off to eat more for a few weeks to train the muscles so you can get in shape faster afterwards.