There are a ton of horse trails within an hour Frederick. I live near Gettysburg and most of the trails I’ve ridden on are in MD. Definitely check out the TROT website for the horse trail list. Is Gambrills sort of off of route 40? Or is that another park? Whichever park it is, I did hike some of it and parts were pretty rocky and there wasn’t much parking space for trailers.
Liberty Reservoir is very nice but the parking I’ve used isn’t great. No official horse trailer parking.
No one mentioned Little Bennett yet. That’s a regional park off of 270 a little below Frederick. They have a nice set of multi-use trails with tons of parking. I want to say like 15 miles of trails?
Sugarloaf is also below Frederick. I’ve never ridden there, but I did check out the parking and like a half mile of trails a few years ago (both looked fine).
Gettysburg is within an hour of you. All-weather trail footing on most of the trails, so a good “wet-weather” place to go. Of course there’s the draw of the history there. Trails are pretty simple, but maybe a bit scary to horses that aren’t used to traffic and strange looking fences and rocks/monuments. Parking is at McMillan Woods Campground. Parking on weekends during the busy season can get a bit hectic if you don’t get there early.
Certain sections of Michaux State Forest in PA might be within an hour of you. Parking and trail conditions depend on which section you go to. Trails are basically endless though.
Patuxent River State Park is off of 70 near Woodbine. I rode there once and really want to ride more there. Tons of space for parking.
I rode at Morgan Run once, but rode onto the trails from private porperty and never saw the parking set-up. Trails seemed nice.
Union Mills, or Hashawa as my group of people call it, is nice. I wouldn’t recommend taking an out of shape horse there. Lots of hills. There’s two parking areas, one off of Saw Mill Road, and the other off of John Owings Road, if I remember correctly. Most horse people seem to use the Saw Mill Road parking area.
There’s another trail system below Frederick that I can’t think of the name of. I walked some of it, and it seemed ok. I think it was only like 6 mies or so, so I never bothered to go down there when there was better options closer.