Fastest way to get my chops back?

I was a trumpet player for about 10 years, quit for 2 or 3, and now I'm trying to retrain myself for a spring concert. The conductor wants me to try out in a few weeks. My range and tone are good enough, but I can't play actual music for more than 10 or 15 minutes. What's the best way to get my chops back into shape, and how long should it take?