run a higher gear than you would normally, or focus on sprints...your legs will fill out nicely at that point.

My legs are pretty beefy from my work commute, since I'm usually sprinting on my way there (due to me never being able to leave the house at a reasonable time)...I don't do the gearing trick though since I deal with headwinds on my trip...

Basically do something that will make your legs give out way before your lungs, and just keep going.