I'm not sure about a place with hook-ups and the trail mileage you are looking for but if you're willing to move around, there are lots of great places to ride in Ga. We did a trip a few years ago and hit 2 of the Ga parks and one of the Forestry Trails. All were very nice with at least about 15 miles of trail or more. They are not terribly far apart either if you don't mind a little drive.  We went to Watson Mill Bridge State Park first. There are only about 15 miles of trails there and we rode almost all of the trails in one day. They have a very nice campground and nice stalls for the horses. A friend of mine likes to go there in the summer. They ride one day and then swim the next. From there we went to Willis Knob which is actually the Division of Forestry. They have a 15 mile loop but you can cross the river into SC and ride the area on that side as well. I don't remember the name of the one on the SC side. There's camping there but it was primitive. There's quite a bit of elevation and it's very pretty. Crossing the river is pretty cool too. We stayed a couple of nights there. They have a very nice campground with hook-ups and poles to tie out.  From there we went to Hard Labor Creek State Park. They have a very nice campground and barns as well. Again, there's a 15 mile loop but they also have a shorter loop we did not ride and I think there are a lot of dirt roads you can ride there as well. We spent about a week or so traveling from Florida for that trip. We were supposed to start at Hard Labor but they were closed because of the winter storm they had just had. We started our adventure at Watson Mill instead. That trip had great riding but it was certainly an adventure! I hope this helps you. It's been about 3 years since we did this trip so some of the trail mileage may have changed since then. I hope you have fun wherever you go.