20 minutes is too long for an interval. I've done interval training with trainers who know this stuff. We generally work up to our max - not just go flat out - for 3 to 4 minutes. Then you test your max for 2-3 minutes (it takes me 4-5 minutes to reach my max, though) and then slowly cool down. Rinse & repeat.

The size of your heart matters. Smaller hearts have more rapid heartbeats (think of a bird). Your actual conditioning, as Lizzy said, matters too. So, use the 220-age as a very rough guide.

Does it matter in your riding? Only if you're doing LSD rides (LSD=long slow duration) or if you want to do intervals. As far as power output, no. As a trained woman, you can probably ride at 70% MHR and smoke most other riders.