Here's my back ground.
Had lessons/owned ponies from age 9-15
Stopped when I started going out and meeting boys 
Had a baby age 24 and having missed it forever, plucked up the courage to start lessons. Had 6 private lessons and just as she said she'd be more than happy for me to join for hacks my DH was in between jobs so couldn't really afford to ride what with baby etc.
Fast forward to now age 29. Financially in a far better position so luckily that's not an issue. However my fitness is. I've made a deal with myself to start doing a bit of exercising to help for my first lesson which I plan to do the weekend I turn 30 in nov. What could I do? I'm 14 1/2 stone
and I really did struggle when I went back after my baby although I was probably heavier then. Rising trot kills me off. And I don't want to go and be embarrassed. Any advise?
The real goal is to get where I need to be by 35 and buy myself a horse.