Try pedaling in your biggest gear at about 50-55 rpm's for about five minutes at a time. If you are inside on a trainer, it simulates climbing. If you ARE climbing, try the big gear, but at low rpm's for short periods of time. This will build strength and will not destroy your knees. Make sure you keep rev's low and push whatever gear you can for the full five minutes.
To get the max out of this, you have to have a very relaxed and still upper body. If your torso is bobbing all over the place, you lose the impact on your legs and back. If you are too tired and are all over the place, stop.
This will not get you huge thighs, but will strengthen your legs a lot.
Low rpm's for short periods, like five minutes with and equal period of rest between efforts.
It's a climbing workout from Chris Charmichael's Trainright company, one their coaches use for what you want.