I ride for fun, for exercise, to stay in shape or just because I feel like it. On Saturdays, I ride 50 miles. On weekdays I work out in the gym on a spin bike early in the morning. I used to ride with lights in the predawn but after getting hit head on by a traffic safety instructor, my wife put an end to that practice. I do a few centuries a year.
I used to ride considerably more but my schedule has changed.
The thing is that a cyclist can ride just about as far as they feel like riding. It is a great endurance sport. Because it is low impact, you can keep it up for inordinant periods of time.
You can even ride each and every day. The thing is that if you do that, you have to do easy rides at least every now and then.
Something to think about though, any exercise that you actually do, does you a whole bunch more good than the "best" exerise that you don't do.