Si wrote:.. if you are saying that you can't hang on to a group that is doing 16-17mph, and you don't have the time to improve your fitness, then if it were me I'd have to ask myself if I really wanted to go on this ride? You may be better off just going by your self or finding someone of the same fitness as you - thus going at your own speed and enjoying the whole thing a lot more..
I agree, the "100 miles in 6 hours" speed is stupidly fast. I've been riding long distances for 5 years and have a carbon fibre fast long distance bike but would find this sort of pace difficult. Difficult and tedious. It would be 9 days straight of exercise rather than travel.
If I assume that the distance for the LeJoG is 850 miles and that this group is doing 100 miles a day (in 6 hours) then it will take 8.5 days
If they did 90 miles a day in 6.5 hours then it would take 9.5 days but the speed would be less than 14 mph, still fast for you- this would be ok for me
In your case and for most generally fit riders with reasonable bikes, 70 miles a day in 6.5 hours with an average speed of just under 11mph +a couple of rest days works out at 14 days. Why kill yourself with a stupid schedule? You aren't setting any records