Well, besides more riding, cardio and diet are the two biggest things that will give you more energy and stamina. Muscular endurance will likely only come from more riding. Swimming is an excelent cardio workout for those who hate running, much like myself.
At 6'2" 225lbs sounds about right. A taller seat foam, bar risers, and lower footpegs would do wonders for your comfort on the bike. I'm 6'1" and have done that to my YZF and it feels much more comfortable/easy to ride.
The more you ride, the more comfortable you will be on the bike.
The more comfortable you are on the bike, the more controll you will have over the bike. The more controll you have over the bike, the easier, and less tiring, it will be to ride. Let it flow and stay loose. If you are fighting the bike for 2Hrs, it'll kick anyone's ass, even someone who is good.