If all you want is more distance ("touring" cyclists often fall into this category), then just put on lots of miles.
If you want all-out speed, then there will be some degree of muscle soreness/pain involved, depending on what aspect of speed you are working on. I found the most painful excecises for running and cycling is sprint training. Of course when I used to run, the sprint training was 1/4mi full-bore, 5mins rest, 1/4mi full-bore, 5mins rest for 6 reps. My cycling sprint training follows that, but is 1/2mi, 10mins rest.
There IS a difference between pain and injury. They are different degrees of discomfort. Stiff legs, well that goes with the quest for speed. Actually I get rubber legs after a good session.
And yes, athletes get joint pains as time goes on, it's part of the price to pay.
However, even in a job, one might get joint pains....look at an assembler. They may get carpal tunner or some form of RSI from using a screwdriver all day long, and only that screwdriver. Same thing applies to someone that works long and hard on the bike.
Taking it easy does have it's benefits, and trust me, I do take it easy sometimes. It's nice to see life below 20mph sometimes
