[QUOTE Went riding for the first time in ages. Im so out of shape! The mare they put me in was really hard work![/QUOTE]

No doubt you did the sensible thing and went to a riding centre. The horse was carefully chosen to be safe for you to mount up on. Your legs are weak, your hips are weak, your thighs are weak. You'll have trouble sitting up straight. You need to get those muscles back in shape.
Look around locally and see if you can find a good Pilates instructor.
Pilates is all about inner core muscles - those that you need to ride.

It is the horse that will get you back in shape. If you persist and get to ride once a week - it will all come back.

Enjoy - it is fun and good for you

B G