My first suggestion is to make sure your friends understand your situation. This tour may take you considerably longer than the more seasoned riders. If they are fine with that and are willing to slow up to have your company on the trip than do it.

My second suggestion is to get off the bike and walk every 10 miles. EVERY 10 miles. Walk during these dismounts, don't sit. You may want to do this after the first 5 miles, too. It will help your legs and your butt to walk. I do this on every ride I do over 10 miles. Sometimes I'll walk a tenth of a mile, sometimes a half mile. It depend on how long it takes to stretch out a bit.

Drink lots of water.

Sounds like fun. What trail?