Talk to her. Don't e-mail her. When you see her tomorrow, say the niceties, and then ask her about her horse. People love to talk about themselves (and what they have). Ask her about her riding experience, etc. Eventually, ask her about where to hire or borrow a horse in your area, because you are interested in getting back into riding.
If she is noncommittal, then you know she probably isn't interested in a riding friend. In that case, you haven't lost anything; you may have a bit of hurt feelings because she isn't interested in you, but most people aren't interested in you (or me!) anyhow; they're interested in themselves. On the other hand, maybe she will be glad to ride with you. You won't know until you've talked to her.
Friendships take lots of time to build. This might be a start.