Another tough dressage lesson

We worked 40 minutes straight yesterday - hard work, and almost all of it at the sitting trot.
The "progress" news is that I can sit the trot through most of the movements for almost 40 minutes. Not elegantly, but a couple years ago, I wouldn't have believed it was possible.
We worked on 10 meter circles, changing direction, shoulder in, and a lot of leg yield. The leg yield was bend him OPPOSITE the direction we're going, and use outside rein to make sure he steps over enough with his hind legs.
The other tip John had was rather than pull my leg back for a stronger aid, turn my toe out and use the spur instead.
For trot, he said even though it feels like I'm sitting on a jackhammer, at sitting try to think left right left right so my hips swing, and at canter think sit glide sit glide. Thinking those helped a lot.
In other questions, John said a supplement should include glucosamine, but that I can try quitting smartpak for a month and see if it makes a difference.
He said that he does a "hack" day after a condition day, which is 20-30 minutes of walk and then a little bit of trot work.
He says sometimes he conditions in draw reins, and that I should be working Duke round, not letting him go all flat.
And for schooling at Spokane, he says ok to drive over and school xc same day, so long as Duke has several hours rest in between. He says if he goes lame from the drive/xc/xc/drive in two days, he was going to go lame in competition season.
I'd say it took until about minute 25 or 30 in the lesson before Duke started carrying himself and quit hanging so heavy on my arms with his head. John even tightened both his nose band and flash because he was gnawing on the bit so much.
John says none of us are working hard enough without the early shows, and that Rebecca is going to come up on his fast, so we need to start working much harder.