Oh do I HEAR YOU!!
I, too, thought I was in pretty good shape cardivascularly...walking 2-4 miles at least 4 d a week, doing sit ups, lifting 15lbs, push ups, etc...NOPE! Riding has challenged me physically (& MOST CERTAINLY MENTALLY!) more than anything after MY 15 year break (rode17 YEARS until quitting @ age 23)...I'm 39, but many of the earlier days (1st 10-12 lessons) I couldn't post more than 3xs around the arena with about 20" between EACH ONE TIME AROUND, for the whole HOUR of my lesson and would become very tired simply trying to maintain forward impulsion to the bridle AND POST on a "more challenging to get moving"/read:slower ) horse...my steady starter mare, Fancy.
Add to that, I have very short legs, so wrapping/draping them around her was NOT A POSSIBILITY until I became secure enough to ride sans stirrups. PLUS, my instructor does NOT BELIEVE (Arghhh!) in even a plastic roller snub spur (which has no possibility of of accidentally "poking" OR A DRESSAGE WHIP OR CROP)!!.
She is of the school that one "builds" enough lower leg strength to squeeze and release, no "kicking"...no real even "tap-tap" w/ heels or side of boot unless horse REALLY NEEDS TO WAKE UP...AND...well she's ridden everything and NEVER EVEN OWNED spurs or a whip thus I shan't either! Many days, between the confusion ("bend her WHICH DIRECTION AND KEEP HER HEAD POINTED WHICH WAY? AND OUTSIDE LEG, INSIDE REIN TO STAY ON THE FENCE??AHHH!!) and being about to pas out from "post up, squeeze, sit, squeeze, up, SQUEEZE --DA*?!IT--SIT, SQEEEEZE..."! I thought "well, I don't have to worry about dying in a spooking/throw/bucking accident ...I'll definitely die FIRST OF OVEREXERTION"! Haaa!
It finally, slowly improved (thank you, G-d!!) and now can ride a 90" lesson and be PRETTY ORGANIZED, in control, AT least till the FINAL ROUND OF NO STIRRUP POSTING. Then, I'm STILL a sweaty, panting mess! :0)
I promise you are NOT ALONE! AND, doing 13 things at once with minimal oxygen will soon become MUCH EASIER! I applaud you for sticking with it after so many years off!!! Now. if I could just BORROW YOUR CROP?? Ha. Sorry...just jealous of your assistive devices! :0)
Best 2 you!! B2H
"I'm too busy working on my own grass to notice if yours is greener"