A 10 min ride is a warm up on a bike and 20 min wont do very much either. It depends on how hard you ride and how long. You want to take the first 10 min easy to warm up. after the warm up you want to pedal at a high cadence 70-90 rpm's with a lite pressure on the pedals. You always want to spin with a lite pedal pressure regardless if you are going up hill, down, or on the level. You want to work hard enough to get your breathing to the point where you can still talk but not sing. You want to ride for distance and don't worry about the speed. You should start seeing results in about 3 weeks You want to ride as long as you can and add more time and distance every other day.
