Thank you both! So so helpful!
She pretty much said it all but I'll add a little! Long reins allow your horse to stretch down. I like to do spirals, and trot poles to help as well. It takes a lot of time for horses to figure out what you want and how to relax and just be comfortable with what you want!
On top of lots of circles, changes of bend, lengthening, and shortening, something you could do is my favourite exercise, which I like to always do for the last two minutes of the ride: once the horse is using their hind end and pushing into the bridle, I slowly let my rein out inch by inch, applying more leg, going as far as possible, while still using the muscles in their back. This lets them stretch their back, neck, and leg muscles. Then I incorporate large circles, causing them to bend and stretch, while still pushing forward on this long rein. If they ever lift their head and start running through the bridle, then I gather my reins and restart.