26 x 1.5 Panaracer Pasela ,T-serv, or Continental Sport Contact for the front and a 1.6 Marathon Supreme on the rear. You'll probably have to replace them before 6000 miles but it would be a nice beginning. There are heavier and thicker tread tires, choices, choices , choices.

If you're riding with lighter less hp riders I'd suggest putting one of the Sport Contacts on the rear and the Pasela on the front of their bike. If one of the riders is a heavy person put this on the front and some thick tire on the rear like the Continental Travel Contact. A heavy long lasting tire like the regular Schwalbe Marathon is fine for a heavy person but if you're riding with light people there's no reason to put a heavy 700gram tire on their bike.

http://www.rei.com/product/711641