Bonking after the first 5 - 8 miles sounds pretty nasty. Sounds like you're starting off too fast to keep up with the rest of the group.

Even though you may have many miles under your belt it depends on the speed you normally ride these at. My boyfriend isn't very fast but he can ride for hours - slow & steady.

Maybe you could ask the group to slowly build up the speed over the first half hour of the ride rather than ' going it from the gun.' Alternatively maybe you could ride with a slower group for a couple of rides and see how you get on.

Good luck