Unfortunately there are only two things you can do for this: go slower, or keep doing them until you get stronger, which will help you go longer. I assume from the other thread you created that you're trying to pass your military PT test, but you don't have a lot of time before the test. This makes the second suggestion moot.
At this point, trying to train yourself to kick out more situps when your PT is coming up in the next couple days is self-defeating. You will show up to PT in the same condition you are in now, so the best thing you can do right now is rest, and prepare yourself.
After your PT test is over, you should develop a training schedule so that you can meet your goals in situps and the run next time around.
Good luck to you!