1) Buy some moleskin. That stuff is AMAZING for stopping blisters. As -soon- as you start feeling a point is rubbing, slap a piece of moleskin on that sucker.
2) I doubt you'll need to stop to catch your breath - that's what lines are for!
3) Being female, you will probably think you look ridiculous in a bathing suit whether you do or not. (I'm right there with ya, sister.) Fortunately, the only perception that matters is your own - so own that glorious body, girlfriend!
4) I feel pretty certain you will fit on Mine Train. I have to admit, my hips and thighs were pinched, but it's a short ride. Don't push the bar all the way down when you get in. It will go down as much as you push it, so leave yourself a little wiggle room. My ex was big - not so much obese as very wide-bodied... broad shoulders, broad hips, and we rode it together. It was cramped and there might have been a suction noise when we got out (ROFL) but I think you'll be fine.

Generally speaking, if you can fit in a modern airplane seat, I think you can default ride anything at Disney except Sum of All Thrills - but that's going away, right?