So, I got in touch with the organizers and they are working with OCTR (Ontario Competitive Trail Riders) to set this up. This will be a training race only, with very easy trails and 4 hour "expectation" to complete 25 km. That's nice because it means I don't have to fit train my horse much. She could do that in 4 1/2 hours now without a hitch. The most difficult part of the ride will be a couple of km in town on pavement! That certainly will be the hardest for my horse, as we very rarely travel on pavement. There is always a shoulder or ditch to ride in where we go. So, I guess I'll head out to the paved roads and let her do some miles on the paved parts.
Next year they are hoping to do 25 - 50 - 100 km races and maybe a "set race" where you are given a speed goal. Cool. I may have found a competitive sport I enjoy. I'm looking forward to my training race this fall. I hope it all pans out for me and that life doesn't interfere beforehand!