I'd get her checked out by a vet. The tying up after 30 mins of work freaks me out. But from what I've heard a lot of different things can cause it, so it may have been something else that caused her episode and hopefully you'll never see another. I think you can test for PSSM too
Otherwise, I would just take her on long walks w/ a little trotting and keep working it up. She should get into shape really fast. If not I'd guess there is something actually wrong with her.