Questions from a newbie exerciser

Yippee! So I got up this morning and decided to take the bull by the horns and try this exercise thing. I have dreaded exercising but my surgery is in a couple of weeks and I would like to build up my oxygen levels. (I'm a singer so my lungs are good but I thought it would be a good start)

Okay, I decided on riding my bike. I rode for only about 20 minutes and didn't go very far. However 10 minutes into it, my jaw started to ache and I tasted blood in my mouth. Did anyone else have this experience when they first started out? I'm also really energized... Does that mean I didn't work hard enough? I know it's only my first day.... still if ya'll could help me answer some of these questions that would be great!

Thanks for reading.