Interval training is a huge way to get your endurance up. Get on the tread mill or eleptical or stationary bike set it for as fast as your able to walk or ride. Get your heart rate up. Get off the tread mill (keep it running) then do as many push up as your able to in say a minute. Then get back on the tread mill or eliptical for a min. Then get off and do as many sit ups as your able to in a min. Get back on the tread mill for a min. Then do your push ups less 10. you can see the cycle Im getting at. Do this untill your down to 10 reps on your push ups and sit ups. I would say do your interval training 3 days a week. Say Mon, Wed, and Friday. On Tues and Thursday you can push Iron if you want. Or just do regular calistentic type work outs. Swimming as mentioned is huge. No better full body work out than swimming. You actually use every major muscle group swimming.
Your diet is huge part of your endurance. Like was mentioned lots of protein. That doesnt necessarily mean only red meat. Fish and chicken have lots of protein. Salmon is really good for you. Tuna is good but get the stuff packed in water. Chicken if your worried about fat stay away from the dark meat ie drum sticks and thighs. Breast meat (white) is better as it has a lower fat content. Dark grean veggies broccillie, spinach, green beans ect. Stay away from high starches (rice, french fries, pasta) and high carbs (white bread) Whole wheat is best. Unless your carbing up the night before a race, they are best, only in small portions. Water lots and lots and lots of water. On an average day you should consume atleast the minimum of 8 8oz glasses of water or roughly a half a gallon. Yeah I know it sucks at first cuz your having to pee every 20 min but your body will get used to it. Be careful with the water as you can over hydrate. Anything more than a gallon a day and your just wasting water unless you really need it. If you feel thirsty your already dehydrated.
Now for the painful part. Stay away from the beer. Its high in carbs. Wine is ok but stay away from the white and pink wines. Reds are the best for you. Soda's are a huge no no. All that extra sugar is just going to make you peter out fast. Energy drinks are another huge no no. Sure they pump you up but are loaded with sugar and other stuff that I cant pronounce. Even more than a large travel cup of coffee isnt all that great for you (but its my vice).
Just my 50 Cents worth.
Matter of opinion, I'm no where near to where I want to be at, thankfully I've still got a month or so to work on it before 
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im talkin hare scrambles lol. i can go flat out for an hr its the 2nd hr that kills me.
