Thanks for all the helpful info everyone! She will canter on a lounge line when asked, no problem, but it's when I'm riding her that she doesn't really understand...I'm using overexaggerated leg cues (moving the outside leg reeeeally far back, asking with the inside leg and outside rein), but the truth is that:
1. She was more of a pasture pet before I got her, and was just used as a trail horse when she was ridden &
2. She is overweight and I think that she definitely needs to build up some muscle first before she has enough balance for a nice slow paced canter.
My trainer rode her the other day and tried to ask for it...she basically did the same thing, ran into it. He got her to slow a little bit but then she just broke into a trot. So I think we'll just need to keep her working at the walk/trot awhile first to build up her balance again. We've been working on her circles in the ring to help with that, and I think she'll get it eventually. He mentioned to me that I should just lounge her every now and then (instead of riding her that day), since she'll canter on a lounge line for me. Then she will be getting some of her balance back that way too.
For now, I'm not going to push her, because as a couple of you touched on above, I don't want her to learn any bad habits by just running into it and/or me having to make her stop. I agree with both Sonny and Appylover above in that it's probably not a good idea to let her run into it because then she'll want to do it all the time instead of a nice transition, but that I don't want to allow her to stop altogether either and learn that she can just get out of anything I ask her for by stopping. I think you both have some very valid points there!
Thanks everyone, I'll keep you posted on her progress!!