Whenever I go to work functions, I wear normal slacks and bring a change of shorts. If it's a country club, they have a locker room and you can change in 3 minutes. If not, you have a car and can't tell you how many times I've changed in my car this year (tinted windows and a minivan help).
As far as being new, I think it's all been pretty much said. I'm far from good but if I play with people I haven't played with before, I tell them that I am better than a lot of people and a lot of people are better than me. That I'll hit some good shots, but some bad as well. I prefer to play better than I say, not the opposite.
Pace isn't as big for me. If I know someone is newer, I'll cut them some slack. It's looking for balls that bugs me. Play your ball and your game, everyone was as good (bad) as you at some point. It's looking for a ball or two every hole that can get rough.
Lastly, someone mentioned the self-deprecating jokes earlier. A couple are ok, but don't go overboard. It does turn to pity after a while. Make a couple of jokes and if you are playing really bad, halfway through the round and at the end, say I appreciate your patience today and I enjoyed playing with you. The worst (and this doesn't sound like you) are the ones who can't come within 50 yds of the fairway and say they used to hit it over 300 yds every time with a nice little draw.