Dietcoke.....you're getting some good advice here (and not so good !) but just remember the choice is yours and I know you've got the monkey on your back but take your time to make the right decision for you.
In my experience, rarely someone's first time with a woman is that great. Mine was terrible ! Don't expect too much. Get some of the practicalities out of the way beforehand (learn to put a condom on if you haven't already!) and don't try and copy what you might have seen in porn because it's not going to happen like that first time.
You might think your cowardly going with a WG first time instead of a civvie but I actually think it's quite brave. And whilst you say it's a one time thing, you might find you want to return.
I would suggest you book and hour and don't jump into the sex straight off. Have an idea in your mind how you want it to go, what you want to do with her and what you want her to do to you, and if the sex is over in 5 seconds that's fine....u might be up for round 2 !
One word of caution though, if you think you're going to regret it in the future, think hard before you take the plunge.
Good luck my man !