Thanks for all your comments and words of wisdom!, it is not a race for me and more a personal challenge as mentioned above, like I said not very fit at the moment and just looking for pointers really.
I used to race and time trail but that was twenty years ago!.
There is no way I could average 16/18 mph at the moment more like 12/13 mph and could probably do 20 miles at this pace with my heart rate through the roof , the course is 100 miles with 1998 metres of climbing, not sure if this is a lot or a undulating ride?, you have eight and a half hours to complete it so no need for 20 mph averages!, although I would like to do the best I can do while raising some money for charity.