When I started cycling, my right leg was doing almost all the work, and my left leg was just being taken along for the ride. Over the past few months, I've noticed my left leg taking on more and more of the effort, and now it's split almost 50:50 between them, like it "should" be. And it isn't just stronger for cycling, it's better for walking on, too - more able to hold my weight as I go down the road, less likely to roll at the ankle or buckle at the knee.
The other day I went for a ride for pleasure, and was in fourth gear on the flat road in Ham. Today I went up The Hill That Used To Defeat Me - in second gear, not first.
I like cycling because I can see improvements in my strength and stamina almost instantly. In a week of just riding to the supermarket & back, I see more changes than I did in two months of going to the gym three times a week. (And that was a specialist program devised by a trained professional in accordance with my body's quirks.) Unlike the gym, if I have to stop for a week because of busyness, it's no problem: I just pick up where I left off. More importantly, this doesn't feel like exercise - it's not boring or pointless, it's just a way of getting places, and fits into my normal life. The only specialist equipment I need is the trike and the helmet, and a lock.