OT: How long to get tired playing poker?

[Indiana]

Poker is going great as I really enjoy the game and make decent coin from it. I've recently controlled my diet (more salads, less carbs and meat) and its helped a lot with my ability to think at the tables. Further, I've been exercising twice a week and that's helped too. Thing is, I used to play much longer hours a year ago w/o fatiguing and now I cannot push myself to be at my best for more than 3-4 hours a day. What if anything do you do to help yourself with the endurance factor? How many hours can you play per day?

For reference, I'm 30, 6'1 185lbs. Always been in decent shape and I do pwn the weekly recreational bball games so I'm not used to tiring out like this.

Indy