Sheila will hopefully chime in here, but I can tell you what I did, which you can of course take or leave!
I didn't start modifying my activity level until WELL into my pregnancies - I listened to my body, and modified when I started to feel a bit off. Specifically, I ran regularly, at the same pace and distances that I was running pre-pregnancy, until I was just over 6 months pregnant with my first. Then, I started to just feel a little "weird" running, so I backed off a bit. Others don't ever have this feeling, and they run right up until they deliver!
I certainly wouldn't change your behavior before you even know you are pregnant! Even if you found out that you ARE in fact pg (good luck on that front, by the way!), I'd still run the race!
I found out that I was pregnant with my first literally days before I attended the spring '99 Cathe roadtrip. The only thing that changed for me that weekend was that I drank decaf coffee, rather than regular! I went at all the workouts just as I had the week before. I highly recommend the book "Exercising Through Your Pregnancy" - it should make you feel better about working out throughout pregnancy.
Good luck with TTC, and good luck on your race!
m.