Wallaby, we are in the exact same boat (Joe even looks a little like lacey :P)! Lol but I'm a college student, too, and my horse Joe is getting older - he's 20 and although that isn't as old as Lacey, he ran barrels up until a few years ago and is beginning to show the signs of arthirits is in right hind leg.
I, too, had the vet out last summer, but couldn't afford to get any xrays done. Joe seems stiff when I work him lately, and it's gotten to the point that I'm scared to do anything with him because I don't want to make him feel worse.
Anyway, I don't have much advice for you at the moment because I'm going through very much the same thing, but I will tell you that I've picked up a book called Equine Fitness (by Jec Aristotle Ballou) and it has a lot of great information about conditioning a horse and also many helpful hints about working with senior horses.
Here's something I found really interesting from the book:
After a long phase of walking and loosening up in the arena, most older horses warm up best at the canter because it is less strenuous on the joints, ligaments, and feet than trotting. Cantering your horse with a light/slightly forward seat might be best for him.
I haven't personally tried that yet, and although I never would have thought of cantering before trotting, the idea does seem to make sense! Just something I thought I'd put out there for you in case you want to give it a try. If you do, you should let me know how it goes! :)
I'll be following this thread - I'm very interested in other people's idea about this. I do wish both you and Lacey the best of luck (she is such a cutie, by the way)!