Johnny L MacPherson,
Congrats on the gig! Now that you've put your name into the hat, you'll probably start getting some more calls, even if at first it's to sub for someone, or to make something on short notice, when the usual cats are already working. So I'll pass along some advice that someone gave me when I was getting started - don't turn down a gig because you think you're not good enough yet.
Gigs are an impotant part of the learning process. In a way it may accelerate your growth, because in the "baptism by fire" scenario, you may be forced to get it together faster than you would by yourself in the comfort of the practice room. Recording the gig is a great idea, so you can go back and learn from your mistakes and reaffirm what you did well.
I found a good tip about walking and note choices on bassist Tom Warrington's site.
http://www.tomwarrington.com/ in the section called warringtonisms. It is geared toward high schoolers, but it can be applied to a tune that you're not familiar with, or one that is faster than you're comfortable playing. In a pinch, it could help you get through such a tune until you have opportunity to work out something more advanced.
Good luck!