Standing with your toes on a step and your heels hanging of the edge, stretches the muscles... I suppose running and swimming would also benefit, leg strength....
There's some good books out on rider fitness.... maybe you could borrow one?
Don't know what else to sugest...
I've found my lower leg has got stronger through walking more...
I use a weight centre thing and i stand on a bar, with weights on my shoulders so my legs are slightly bent. Then do reps going up and lifting the weights then going down and holding it.
Or just stand on the edge step and push your heels down then raise them up