I have figured it out over the years that if you want a "slower" horse, you have to do alot of trotting and cantering...eventually they figure it out that it's SO much easier to go slower, and conserve their energy rather than bolting and running hard, especially since you are going to continue til they slow down anyway.
My mare had a terrible habit of just rush, rush, rush...but not now...hahaha...once she was in a bit better than paddock condition, I would just do a lot of trotting; circles, coasting around the arena, some serpentines, etc...she eventually just started slowing down on her own, instead of breaking into a super octane trot every time! It's amazing how having a horse coast around in a gait and keep him going beyond when he is asking to stop, will give you a more balanced, and more even paced horse. And then we started adding a bunch of canter work, as well, once we had the trotting sorted out a bit. I still do a lot of trot and canter work, mainly because that's what's going to keep her in decent condition in the first place.
I would say as long as you do a decent warm up, and especially make sure she is breathing normal before taking her back to the barn, there is NOTHING wrong with a good workout. Remember it's miles that are going to make the horse a good riding horse, not just going out and doing a tiny ride and putting him up.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."