Ya gotta crawl before you can walk. You gotta walk before you can run.
Shooting a bow is a physical activity and you should not expect to instantly become an expert. Start slow and easy, work yourself into condition. If you're getting tired after only 12 arrows, I'd say you need to back off the draw weight, substantially.

Don't go hog wild on trying to extend your range right now. Practice at hitting the spot at 20 yards until you are confident you can do it every time. Then, in the woods, work at getting yourself within 20 yards of the animal before shooting. The average distance whitetails are taken is something like 18 yards anyway.

When you are confident in your shooting at 20 yards, then back up to 25. Practice there until you're confident. It might take some time, but don't rush things. Keep increasing your distance by five yards as your confidence level increases.

Back to draw weight. I suggest dropping the bow to it's lowest poundage right now. Shoot every other day, making sure you stop when you feel the slightest bit tired. Once a week, increase the draw weight by 1 turn of the limb bolts. ONLY 1 TURN! You probably won't feel the difference but that's because you've gotten stronger and your drawing technique has improved. If it does wind up feeling too heavy, then take it back down and practice another week before trying to raise it again. Keep raising the draw weight by 1 turn each week or so until you hit the poundage you want to shoot.

The key to getting off to a successful start in bowhunting, and the very first lesson you must learn, is PATIENCE.