Nothing wrong with being "a quick study" your first time out, as long as she knows to expect it and you're ready to "make up for it" later.
Know how long I lasted the first time I had sex? Two strokes. Probably less than three seconds.
That's right! I've won every contest on the subject I've ever been in. Quickest draw in the west!
And now, 14 years later, I have a loving (and very satisfied) wife who I can make love to for hours at a stretch. I regularly outlast her. So relax and let 'er rip. You'll get better as time goes on.