My commute is pretty short (5 miles each way). I let me mood, physical feeling, and weather dictate my speed. Most days I see my ride as 2 spinning sessions. During the winter my speed is much slower, but studded tires, more wind, more clothing, etc all conspire to make the same ride harder work, yet slower going. Most days I like to ride pretty hard and feel nice and spent when I get to my destination. It feels great and you feel like you are realy helping your body get stronger. Some days you'll feel tired, or are just not feel energetic. I still ride, but may not push it very hard. One a 5 mile ride that may only make a 3 minutes difference, but it that is what the body want, then that is what I do. I almost always feel better by the time I get to work so the ride still has theraputic bennefits. Unless you ride a lot you may also find that at first, you'll be completely whooped at the end of a week of cycling, and you may also feel your legs pretty much all weekend long. After some weeks, the body starts to adjust and the soreness goes away. Then one day you'll look at your average speeds and realize you now ride MUCH faster than when you weren't riding daily.

Happy riding,
André