you can still exercise! lots of people exercise when they are 70!
and yoga should be fine, i don't do yoga, but some of the stuff on insanity requires flat on the belly, and i do all of them.
as for eating, it just sort of depends, at first there will be lots and lots of gas and output, but it will lessen as time goes on.
It depends on what i eat as to how quick it comes out. Like pizza is out before im finished eating, a sandwich takes a while.
Don't worry about being a gasbag! as long as you don't let yourself get hungry, or eat a lot of the major gas foods, you shoudl be find