15mph is a good avg to aim for. Very important to keep your effort steady at first. As said above, start slow, finish fast.
We did Port Talbot to London a couple weeks ago. 198 miles, 13 hour riding time. Real stops are also very good for energy and moral! We stopped 3 times and ate something real. I find that as long as I keep eating steadily after 100 miles I’m fine.
http://app.strava.com/activities/6398868
Have you planned a route? I find nice roads also help alot. Trudging along an A road for 150 miles when you’re tired and hurting is soul destroying! make the route enjoyable and it will feel much quicker.
I’d be happy to join you for the first 50 out of London if you want some company (and a tow!).