Ride a lot and challenge yourself.

You don't need to do uber-long distances. You could get very fast without ever doing anything over a 40 mile ride. Eat lots and lots of hills. A nice steady diet of hills. Hill repeats, intervals. Eat lots of protein and do recovery rides in between your difficult, high intensity rides.

You don't need a power meter, though they do help once you get to a certain level. Just ride difficult terrain and push hard...you don't really need to be more precise than this until you are more advanced.