>>3457
I can think of two major reasons you don't have endurance and get tired easy.
1. P.E.
2. Diet
Do you stretch daily? Yoga? Weight lift? Push-ups? Run? Jog? Walk?
Do you eat fresh, local, organic fruit and vegetables, or processed chemical poison? Do you ingest a large amount of carbohydrates? Do you eat while sitting at the computer on 8chan?
What the hell does any of this have to do with art?
3 hours is not a bad stretch, anon. Most artists work in shifts. Major projects take days, weeks even, months or years depending on the size. Don't get frustrated because you are at this stage, it is one every artist has to get through, or fail. You need to learn to pick up where you left off on a project, to discipline yourself not to allow yourself to start a new piece but go back and work on an old one. Touch it up. Render it a bit further. Take a step back from it. Review it. Look at it with fresh eyes. And keep working on it. It is a skill in and of its own. And it takes a bull-headed determination, which is where the above comes into play.
Self-control is not a thing you can divide up. You are you. All of you must be working together as a whole or else you will sag. So if you are not getting in shape or staying in shape, if you are not eating right or continuing to eat right, then you will sag physically and mentally. Then your art will suffer for it and you won't know why. You won't have joy in art any more. It will become work, a chore.
But when you're in shape, when you eat right, you sleep better, you feel better, you think better, you like yourself more, you like you life more, everything is on an upward spiral for once and not on a downwards one. You'll see it in your art.
As for where to go, there is rarely a better subject than the human form for general study. It is almost always worth your while. I'd get a book like this and work through it, start to finish until you are done:
http://www.amazon.com/Anatomy-Lessons-From-Great-Masters/dp/0823002810
Keep drawing!