I used to enjoy running, but my knees started to deteriorate and I got so I couldn't run without pain. I'm told it's arthritis. In my hips too.
Then I got into weight lifting and strength training. Did that for a few years, but hurt my back. After the third time doing that (yes, being very careful, working with a trainer, using a weight belt and everything), I had to give up on that as well. Apparently I also suffer from Cervical degenerative disc disease.
I got into kayaking after that, since I live pretty close to the river. That was fun for a while, but I just didn't like being in the water when it's cold, and I found I wasn't going very much because of the time to load-in and load-out and coordinating with other folks if I wanted to put in up river and ride down to another spot.
That was when I got the idea for cycling. Sold my kayak and started looking for a bike. I bought used bikes, tried them out, learned to fix and adjust them, and went through several figuring out what was comfortable. I also found that I REALLY enjoy working on bikes. They are nice little bits of engineering, and you can do a lot of work on a bike without spending tons of money on parts and tools. And I like the whole idea of having transportation that doesn't require a gasoline engine. I commute to work and on trips and sometimes just go riding to ride. I've been to the river a lot more times than I did on the kayak, there are some really nice bike trails there too!
So, yea, I do it for exercise, but also for commuting, entertainment, and general fun. And I've always got a project bike I'm working on.
It did take me quite a while to develop some "biking legs", like months, and I still get that bit of leg strain around miles 2-4, but it goes away after that. Love being out on the bike. I go all over town on it!