During my sprint tri, I was averaging that speed on an upright hybrid commuter bike with fenders and a rack and a saddle with springs. In a skirt.
Trust me, you will go MUCH faster than your 15 mph on race day! The excitement and the thrill will have you zooming!
Use your drops, ride ride ride to get your legs strong, but don't stress about your average. It will be utterly different on the day of your race.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson