+1 on walking. I still try to alternate walking and cycling - I remember an article in Bicycling back in the 1990s, where the trainer for what was then the US team in the Tour de France saying that walking, and particularly hill walking, is the best way to build up your legs for cycling, and that cycling by itself won't do it.

Another good thing to do , that doesn't take up a lot of space and doesn't cost a lot is free weights and a folding bench. I use dumbbells, so the space requirements are minimal. This won't get to you Charles Atlas (who remembers him?) proportions, but it will help you tone, and if you do an upper body workout that focuses on the abs as well as the shoulders, arms and back, it will improve your stamina on the bike. There are a bunch of good dumbbell-based workouts around, and I think they're all about equally good.

If you alternate days between 45 min to 1 hour of either walking (briskly) or cycling (spinning), and an upper body workout with free weights, you'll see marked improvement within a few months.