Dude, do the ride. There is no motivation equal to the motivation you get when you're about to get dropped... it'll amaze you what you are capable of!
Besides, 15 mph isn't really that hard to do - you just need to know you can do it. See where I'm going with this? The other factor is that riding behind somebody is 80% as hard as riding solo.
Bottom line, you know the course, if you get dropped you still get a good workout, so go for it. Allez Allez Allez!