9-10 miles is not a long ride for a sound horse at a walk. That is pretty standard trail ride length for casual riders, many ride longer than that.
I agree that the biggest concern will be potential behavior issues if your horse isn't used to being ridden out alone or near traffic. If you anticipate that being a potential problem, start working in it now.
If the weather is really hot you will want to offer him water along the way. He would likely be fine for 10 miles without water though. If there are no natural water sources on your route, maybe someone can drive a car to bring you water halfway? If you do have water, let him drink as much as he wants. I let my horses graze along the trail - not sure if you will be on roads the whole time, but if you will go by an area with clean grass you could let him stop for a snack.
Last edited by TheOtherHorse; 07-19-2013 at 09:48 AM.