I've been biking for about 5 years, but can't seem to get any faster. I ride about 1,500 miles per year, 20-40 at a time. My endurance improves as the summer goes on but I can't get any faster. What's the secret? I can maintain a 80-85 rpm comfortably but don't seem to have the leg strength to push harder without wearing out sooner. Any tips? I'm 46 years old. I've been reading about interval training lately and want to give it a try.