Learn how to rate the metabolics of your horses. Then you can tell if you are pushing them to hard. A flatland horse may just need more time to climb a hill than a local horse. Just like you. Get them in shape. Get them used to the feed they will be eating on the hunt ( Pellets or what ever you bring) so they don't get a big change in diet.

Learn how to tell if you horse is dehydrated. Look at his capillary refill, tent his skin, how fast does his jugular refill. Be familiar with his gut sound. Then you can tell if you are stressing him.

Your horse will be better off with grass hay than alfalfa. Too much calcium that causes the horse to have a heavy lathery sweat and pee off his hydration. Check into the Endurance horse training. You can learn a lot about conditioning a horse. A lot can be improved with proper nutrition.

I wouldn't own a "nag" ... but my horses do get plenty of work. My wife does competitive trail riding, and I'll be trying it as well next year. They'll be as "in shape" as I'll be.

What feed options are there for this sort of work? I generally just throw grass at them, or alfalfa when they're working harder ... rarely grain. But maybe that'd be needed in the higher elevations.

Hello;
The best thing you can do and something I never seem to find time for, is to condition your horse, by riding him regularly over varied terrain. Just like people, you can't take a fat flabby horse and work the dickens out of them for long. On the other hand, it's amazing what Endurance riders can do with their nags.
Grizz