Alot of good advice offered above. I would suggest if you concentrate on squats, lunges or any of the above mentioned exercises make sure you also include or do variations of the leg curl, deadlifts and good mornings. Road biking is more quad intensive. Adding squats, lunges, etc further concentrates on your quads. You want to keep some sort of balance by also targeting your hamstrings (squats and lunges will work your hamstrings some but not to the degree that deadlifts, leg curls and good mornings will).