...He will let you know when it is too much - quote the DVM Extraordinaire.
I was speaking with her last night...she has been on the on-call shift from hell...and we are trying to sort out when she will be able to make it out to visit us...anyways, we started talking and the conversation went a little bit like this:
Me - Well, I am scared you know? What if I ask him for a 20m circle and it sets us back? What if I ask him to canter in the ring and it sets us back?
Vet: PUSH HIM, he will let you know when it is too much. If you don't push him, he'll plateau and you want him to get stronger.
Me - I'm just scared to do too much.
Vet: DON'T be scared - he is smart enough to tell you when it won't feel right.
Me: Okay, if you are sure.
Vet: I am sure. Have FUN!
So - last night - I was all excited to ask him for MORE in the ring...like more engagement, 20m circles and cantering...but I ended up stuck in the office until 8:30...joy. So we will try again tonight.