Improve your health your a younge dude ,i jog twice a week and can shag as long as im interested .And like others have said break it up ,dont turn into a pnumatic drill pumping away relentlessly, , slow down enjoy .
Yup. Keep fairly fit, and just slow down. Longer/more foreplay, etc. It's totally natural to shoot your load fairly quickly. Your body thinks this is for making a baby, so wants to get the job done ASAP.
For what it's worth, if you get on with someone, just get the first one out the way and keep things intimate, and be a little persistent in getting hard the second time, even in 1/2hr session it can be done. I've done it quite a few times, and I'm no athlete or tantric master or anything. However, if it's with someone who's even slightly indifferent, (and especially if they don't kiss) it probably ain't going to happen.