I competed in my first cross race on sunday in baltimore and I used a 42x18 and thought it worked well. I am not much of a spinner though so it suited me well. It can definitely wear you out because there are times where you get to such a low speed that you have to use a lot of power to get back up to speed instead of spinning back up.

I say just go ahead and use what you got and make changes later if you think you need to. We can make suggestions all night long but the reality is only you will really be able to determine what gearing you are comfortable with.

Good luck and have fun.