If it's endurance, ride a lot. Climb a lot. I always say that endurance starts when you start to endure. A 4-5 hour ride is ideal for building endurance, with at least 50' of climbing per mile. If you can't do that, work up to it. That'll help the weight, too.
Leg extension strength, like the one-legged squats, is helpful, but you need all your muscles, not just the push-down ones. If you want to do the squats, do them off a chair with one finger on a wall. You should be able to do sets of 20. But on the bike is best, unless you've done all the endurance work and the drills and the intervals and the hill repeats and still need something more. Then weight work has its place.