Endurance is a very important aspect of brass playing. Especially for horn and trumpet. Here are a few things you can do that will help you go the distance.
1) Play every day with only a day of rest once in a while. That one should be a no brainer but I very often go to schools where the kids only touch their horn for band practice. They struggle to finish their band pieces and don't know why. Well...
2) Play your etudes a step higher. It'll hurt a bit but you'll be stronger when you go back to the original key.
I tried it a couple of weeks before playing Mahler 7th. It worked pretty well!
3)The Caruso technique. A series of exercises created by Carmine Caruso to develop strength and endurance. I personally do it three times a week as a little workout. Some people say it messed up their embouchure so you'll have to be careful with this technique. I think that if you don't overdo it, it should be no problem. You can find some of these exercises online or buy the book. The rules are: don't force to get higher, don't do it everyday and only once a day when you do it. You should see results in about three weeks.
4) The "BERP". A little plastic device you attach to your instrument and put your mouthpiece in. You can adjust it to feel more pressure on your chops. It's a bit like running with weight on your feet. It's also very good for ear training. They have it for all brass instruments in music stores or online.
5) Practice fast passages slightly slower. You might think that by playing faster, you'll finish the piece quicker and will be able to go further. The problem is that this way, you put more stress and tension on your body and don't allow yourself to breath enough to provide oxygen for your muscles. By playing slightly slower, your body relaxes and you get a better air flow which will take some pressure off your chops and make you sound better.
6)Play half the piece and add a line or two a day. You'll be amazed how quickly you can improve your endurance on a certain piece by doing this. Some of the endurance problem come from the lack of confidence that you'll be able to finish the piece. If you're nervous about it from the start, it'll get worst as you keep playing. If you set a goal you can reach, you'll feel better and stronger. You can set the "finish line" a bit further every day and after a week or two, play the whole piece. You'll still be tired at the end but it should be much better.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.