Various tips I've gleaned online and from my attempts:
- First buy a horse care package at the stables north of Van Horn. A golden stamina core for your horse is very helpful.
- You need to make sure you see the "Van Horn, [Time, Temp]" notification before starting. You get one every time you exit a stagecoach, so buying a ride to start is a good idea.
- For the route, head south out of Van Horn. You can either cross the Kamassa via the rail road to the west, or via the bridge to the southwest (just north of Bluewater Marsh). After that, set Flatneck Station as your waypoint and try to take the most direct route possible without getting into rough terrain. Once three, you'll cross the Dakota via that railroad bridge. Then you'll head northwest, up through Strawberry to the north side of Lake Owanjila. Ride south over the dam, go southeast through Tall Trees, and then it's an easy ride to Blackwater from there.
- If you dismount or fall off your horse, you fail the challenge automatically. 17 minutes is actually more than enough time to make this ride comfortably, so don't feel like you have to be slamming horse stimulants and riding at full gallop the whole time. You'll be much better off slowing down through tight turns (like the switch backs in northern Tall Trees) than risking a collision with a tree.
- Once you've got a good route planned, the hardest part of this challenge is surviving the bounty hunters that jump on you in Tall Trees if you attempt this during Chapters 1-6. But since you're in the Epilogue, you won't have to deal with it.
I'd guess you're failing due to either (2) or (4). Good luck.
Nope, that's exactly what I'm doing. The only thing I can think of that MIGHT be messing me up is Dewberry Creek, but I'm 99% sure I'm not touching any water when I go past it.
I don't know how bad the bounty hunters are in #5, but there's still an ambush in the Epilogue. Usually it's about a dozen Skinner Brothers lining the canyon walls near Tall Trees and then they follow you on horseback through Beecher's Hope.
I'm timing the whole thing on my phone and I'm doing it in about 15 minutes real-world time.
It's not the only challenge that's felt buggy. Weapons Expert 2 never saved after I had completed it, so I was working on Weapons Expert 3 and wasting loads of Tomahawks, not understanding why I wasn't getting credit.