A Race this Weekend?

My friend sent me an email yesterday asking if I was interested in running a 5K race this Saturday.  For some reason I'm hesitating.  Yesterday's run was really hard.  I'm not sure why. 

Dave and I ran together with Maggs in the jogging stroller.  It was a beautiful day out and we had already gone for a walk earlier in the day with a new mommy friend.  It wasn't a very long walk or a very hard walk.  I did run in to my high school softball coach which stunk because I can't stand him.   We were playing outside when Dave got home and he was crabby, and Maggie was crabby and that made me crabby. 

I started out pushing the stroller and immediately my left knee started hurting.  Ugh!  I know I need new shoes, but my knee has not hurt me at.all. in a long time so it start worrying about it right away.  It never got any worse though so I kept going.  My Nike+ didn't work for the first 5 minutes so I had to try to fix it while pushing the stroller and after all of that I was just out of gas.  I was so tired.  Dave was running ahead of me which I shouldn't care about, but it makes me feel even slower.  I mean he is 6'4" so of course he's faster than me.  I took Maggie home a little earlier than Dave and then I decided to run a little bit more, around the horseshoe of our street.  The jogging stroller eventually hurts my shoulder because I can't keep them relaxed while pushing it, especially into the wind.  Dave and I switched pushing a couple times, but that didn't seem to help.  I ended up running about 40 minutes and about 3.6-ish miles (see about about nike+ not working at the beginning so I'm not 100% sure of the total mileage or time) It was slow, but oh well.  I guess I just have to chalk this one up to a bad run and go from there. 

So anyway, I'm not sure if I want to race because I had such a bad run and my knee was bothering me.  I don't want to get hurt.  I'm going to take today off and run again on Wednesday and see if it feels any better, then I will decided about Saturday. 

Do you get discouraged by a bad run?  How do you "get over it"?