Work on 10 meter circles

Today we talked about fitness (human and horse), exercises I can do at work and home, and then did such hard work on 10 meter circles that now - 6 hours later - my butt is sore.
First, I wanted to know why I got so out of breath in my lessons (and sweaty).  John said he gets sweaty too, and that part of it is because I am using a whole bunch of muscles I don't use when I ride by myself (because it is harder for me to just keep going than to rely on John to tell me what to do next).  In a lot of my lessons, he is telling me to adjust things step by step, and that means muscles are always working.  He also said there's nothing else like riding to work the muscles - running and biking both just lengthen them.  He said the closest is swimming.
For Charlie, he said that yes, once Charlie is fit, he can maintain it with just a little bit of work.  Like how I got him fit for the three day, and then he lasted the next six weeks for the rest of the season without me needing to condition him.  He said it is a lot of work to get a horse up to a full format * fit, but then it is just a couple extra minutes of gallop to get them up to **.  He also said it is much harder now, when people don't have access to places like his where they can just go gallop around the 40 acres, then come back and keep jumping.
But yes, they can get dull, so break up the routine of work with just some walking around and chatting before going back to work.
For fitness, I can do the adductor at the gym, wall squats at work and at home (work up to 20 minutes!), and stairs.  But he said on the stairs to first walk on my heels and stretch out the Achilles Tendon so I don't injure it, then do the stairs with my heel hanging off the back.
He also said that my position is not good at the apex of the jump, but my leg is not swinging, and my approach and landing are good, so it was not as bad as I felt like it was.  (I was sure I was doomed from prelim and we were going to have years of basic work before he'll let me go up a level.)
For Charlie being stiff yesterday, he said yes, the long low work was good, and also add a bit of long low leg yield.  My lower back was probably sore from trying to keep my shoulder blades together, and holding the position over the top of the fence.
So today we did a lot of 10 meter circles, at the trot and canter.  Charlie threw one fit, kind of early on, and it was relatively brief.  This was one of those lessons where John gave me instructions at every step, so it was too much for me to repeat at home, but one big takeaway was to push down with inside seat bone with the canter aids (and then, even on a 10 meter circle, Charlie picked it up instantly).  For the down transition from canter to trot, don't drop my hands and keep my legs on so he doesn't plummet down.  Do NOT let my inside hand get dragged down, keep it up.  To have bend on a small circle, make the inside rein a couple inches shorter; then my hands can be even but I have bend in Charlie's neck.
We did 10 meter circle, shoulder-in on short side, another 10 meter circle, etc.  Then we did the same thing but going down the long side without changing anything and going deep in the corners.  John said we had a nice trot in the circle that used to be our lengthened trot.  Charlie tried to twist his head, so I had to put the inside hand up a lot and hold it there.  We did a lot of the work sitting.
Charlie also tried to bulge out the outside shoulder, and I had to use outside leg to keep him from escaping, but then he'd start moving in, so I had to follow it right away with inside leg.  It was a lot of concentration and a lot of reaction and requests all really fast.  Charlie did great (other than the one little tantrum), and it was - as usual - a great lesson.
John said that I can do work without stirrups also, but to be careful not to start pinching from my knee - to remember that exercise about holding the dollar bill.