my guess is that S360 = Short 360degree ride, out and back, or around a lake or valley or mountain. He said it was 45 miles each way, or 90 mile ride in total.

For some that is a day trip, for others it could be a 2-4 days adventure. Not know what bikes you have, and not knowing if you plan to sleep overnight someplace, I'd suggest using whichever bike that is currently in good tune and that you're comfortable on. If the ride is part or all gravel or dirt, or is in conjunction with a group ride of some sort then that may change things. But if this is just a personal round trip ride and isn't a test ride for a planned longer trip, then I'd say just use whichever bike that is in the best condition and that will carry whatever it is that you want to bring along.

How about giving some options on the ride and what bikes you have to choose from to garner a better reply. In any case, have a good ride.