Beginer signed up for sprint distance - need help

I was feeling brave and signed up for sprint distance tri.  I knew I could do all the distances independantly and had been running 5km's in 23 minutes.  Had not swam in 15 years other than with the kids and only biked with the kids. 

To keep my costs down, I have put road tires on my 34lb mountain bike and had it tuned up.  Biked 20 k today in 34 minutes to give you an idea of where I am at with training.  Will I be the only one on race day (sept 19) with a mountain bike and how badly will this hurt me?  My goal is to finish, but I don't want to be embarassed either.

Swim has come along way so far in the short time.  I have swam 6 times, twice per week, approximately 1000m for my 600 m race.  Purchased tri shorts (not yet arrived) and some goggles.  Any suggestions welcomed.

Most of my training is done at night after 8 p.m as I have three kids under 4.5

I have talked to many experianced people so far, but they are so far past what I am doing, some of their suggestions are beyond me and some have been great. 

I need help as a newbie please!