I will add that although speed isn't that important if you can cover the distance at a comfortable pace that is either quicker than your current pace or faster than others in your group it means 4 things:-
1. You spend less time on the saddle each day.(good news for your butt)
2. You get more time to rest each day either in breaks or at the end of the day.
3. If you cycle with the group you will be cruising making the trip easier.
4. More rest means more time to eat which is vital on long trips.