This type of advice is correct, if you're primarily talking about anaerobic activity. When you perform exercise to failure, which I seriously hope doesn't happen to you on a bike (because you then have bigger problems), your body needs lots of rest to recover. If you're a weightlifter, then your friend is correct that you generally need at least a day's rest. It's during this time that your body adapts to the stimulus. The stimulus from a 20 mile ride is not nearly the same, nor is the wear on the muscles; we're talking an aerobic activity you do for an hour instead of maybe half a minute. I think 4-5 days a week is fine for someone starting out. But you should definitely take a couple of days off a week. Your cv system, muscles, joints, and nervous system all need rest. At this point, just back off if you feel tired or unusually tight or sore.
Exercising every day won't make you waste away or anything. Unless you're really sprinting to work, you're primarily working your heart, which doesn't need as much recovery. You just won't get the full training benefit, and you set yourself up for injuries and risk bailing on your exercise plan. You will temporarily lose strength once you get fatigued. Days off also help fight mental fatigue.
If you have to ride every day, and the ride does fatigue you, leave yourself a little extra time say on Wed or Thurs (assuming Wed is the middle of the week for you) and spin nice and easy. Won't get the HR up too high or tax the muscles much. Overall, if you can handle the ride easily, I wouldn't worry.
Last edited by tadawdy; 06-05-09 at 01:36 AM.