Congratulations on your acceptance!
They're not that selective because it's tough to find MCAT teachers. If you have the scores and can speak okay in front of an audience they'll give you a shot.
My first time though the course was pretty rough--it had been two years since I took the test so I had forgotten everything. I had to spend seven to nine hours prepping for some of the tougher classes, but usually averaged three to five. Now that I've jogged my memory, relearned everything, and seen the lessons a few times, I usually spend an hour or two prepping for each class.
To me, the compensation is really good. I teach off-site so I get an extra hour of teacher pay for each class. I get to bill three hours of prep for each class. I can hold pretty much as many office hours as I want per week (2-6 hrs) paid at the teacher rate. Out of class is 7/hr, teaching is 26/hr.
I think teaching rate is determined by education, teaching experience, test scores, and how your trainer thinks you'll do. In my area it starts at 20 and goes up from there. If you want a chill job, get into MCAT teaching and get your pay rate determined based on that, and switch over to GRE or something--the work will be way easier but your pay won't change.