Remember- the Derby is a marathon, not a sprint.
The best method is to maintain a fast trot or slow canter and stay at a consistent speed. Frequently changing between galloping and walking is taxing for the horse.
Mongolians don't sit in the saddle at anything other than a walk. It's best to be up and off the horse's back as much as possible as it will tire them less.
Water your horse
When to come across a body of water make sure to offer your horse a drink. If you're camping out it is of utmost importance that you keep your horse hydrated- in this case he is your responsibility overnight.
If camping out you must allow your horse to graze for an hour before riding.