My friend to have sex for a long time you have to be mentally prepared than being physically prepared. And to do sex for long time does not mean that how many 'pushes' you can make once you have penetrated. Having sex for long time means the over all time two adults can have together, during which there will be foreplay, build up and ultimately the orgasms. Now to achieve this one has to have faith in him/herself. If the partners are really in love with each other, no scary thoughts come to mind about the performance levels. Secondly, partners have to understand what the other one likes more or less so one can improvise and that will come with time. It's a gradual process, like starting with kindergarten and one day achieving the doctorate. So be patient and do this penis exercise : When you urinate, try stopping it mid-stream and then let go as many times you can do every time you urinate. The other one is squeezing your rectum muscles and releasing them. When you do that the penis muscles are exercised too.
Also, when you are new to the partner do not get impatient, it takes time to come to your full potential.
rightly said by vagabond " you have to train your mind to think of other things. If you can distract yourself you should be able to have sex for a very long time."