A couple of things... And this is coming from a similar sitaution.
We are an overglorified garage band for sure, our lead guitarist is by far the best musician of the bunch, and he is fair at best. Rhythm guitar being the worst. And that leaves me in the middle. No comment there aside from I know I need to improve and am working on it at every opportunity.
#1. Practice.
#2. Practice.
#3. Did I mention practice?
#4. Hopefully you have portable gas, or kerosene heaters. Mind you, I don't do outdoor winter gigs up north. Come to think of it, I just don't do winter up north... Sorry, but I know on dreary winter nights here, we use a couple of propane hunting blind heaters not too far away from the stage sides, and play small stages, the heat gets to us.
#5. Like was mentioned before, let your instruments acclimate, and retune, often.
#6. Make sure what you know is well under your belts.
#7. Pick more material that is easy for you guys to nail, and learn it.
#8. If you can get away with it, simplify lines where you can while keeping the groove / feel. Most audience members won't notice, and those that do will think you are "making it your own" OR have some sympathy for a new growing band struggling through. Yeah there will be a few D-bag critics there, ignore them.
I have to assume that whomever hired / invited you to do the gig must have heard you play, so you should be good enough for them. Do your absolute best, and use playing beyond your abilities as a way to stretch to get to the next level. Honestly at best, as a band you become more cohesive and up your game a notch, at worst, the band comes apart because those that aren't serious about it give up, and you have a good learning experience.