Here are the 2 I've been looking at. I know what you mean about bringing our own bike, I'm just thinking for 18 miles is it really worth the hassle / expense.
http://www.sportsbasement.com/Rental_Bike.asp
http://www.bicyclerental.com/submain.php?category=rentals&subnav=fleet
I did find one place offering a Tri bike but I'm guessing Road bike is the way to go, as it's so hilly and technical.