You shouldn't have any problem if you are already doing 20. If you are maxing out at 20, you may be pushing too hard. Figure out heart rate zones and stay in your Zone 2, which is an effort where you could speak a full sentence without have to take a breath. Then, increase you mileage by between 5% and 10% a week. You should always alternate "hard days" and "easy day" and at least a day in between rides. I also do hard weeks and easy weeks. This periodicity is one of the secrets of building strength, speed and endurance. Good luck! P.S. I'm 70 y/o with heart attack at 43 and quad bypass at 64. Since then I've ridden about 8,000 miles with no illness or injury. After about 2 months this season, 30 mile rides with 1,000 ft. of climbing are very easy. My goal is ride my age this year, which will be 71 miles. I may do that in August at an organized event or September on my lonesome.