My biking friend who I have the utmost in respect claims that unless a cyclist pushes hard on a ride - to the point of finishing a ride with some level of fatigue or muscular soreness - that you aren't getting the benefit of building muscles. Thats how I understand him.
My logic would have it though that I can still derive benefit from a "casual" ride.
btw I've done 1700 miles since last May '05 on a Trek 820 ( steel ) mtb - mostly road riding on slicks - and just got a Devinci Podium road bike, Shimano 105 with Shimano wheels. Getting used to the new light ride, which is of course faster and easier ! yes, less exercise so I guess I go longer & faster to make up!
My quick rides b4 or after dinner are 11 miles; and a couple times a week I get in 20-30 mi which really have gotten me to be an endorphin addict