I started commuting a longer (10 miles vs. 3) and then it got cold and suddenly I was exhausted all of the time. The solution: CARBS! I just started eating more carbs and calories in general and it made all of the difference, so be aware of glycogen exhaustion if your ride takes awhile. You might need to eat more, generally a half an hour before you start, especially in the cold. I eat bread and honey in the morning right before I shower. I also always eat something right after I get off the bike to replenish the energy I used.