as mentioned, being younger and in very good shape is a big help, but given that you will be doing a lot more riding, and riding where you are working a lot harder than your commute because of the weight (and I suspect throw in going faster than you may think because of the competition between you and your friends;-) at the very least get a really good experienced look at your bike setup--seat height etc so that there isnt anything really out of whack that could end up causing you knee issues or whatever.

if at all possible, despite your "hitting the ground running" comment, planning the first couple of days with less distance is always nice to let your legs and bum and everything get used to the work more gradually. Depending on how much weight you guys will be carrying and how much climbing you'll be doing right off (not to mention your bikes gearing being low enough) doing a few days of 70 or 80k is a nice way to ease in--plus it will show up stuff that could cause grief.
Hopefully though, doing some longer rides with stuff on your bike about the same weight as you plan to take is always going to be good to really give you an idea of how 80km with load feels, and this is the time to find out stuff like, I dunno, your hands hurt like hell, your arse hurts like hell due to improper shorts or whatever, or a hill is murder to get up because you can't gear down any lower etc etc.

I'm twice your age but when I think of my first bike trip, I really should have done more riding with all the weight on the bike before the trip so my legs and knees would have been stronger.

If anything though, really make sure your bike is setup properly for you. You or your friends don't want any knee issues cropping up because of a bad setup, even at 24.

mostly though, have fun. Think of it this way, proper training before the trip will just mean you'll have more fun, so its a win win.