I'd expect 26" Marathons to last the entire trip, front and rear. If two (or more) of you are on this tour, I'd suggest you take one lightweight folding spare tire as an emergency backup. If you do wear through the tread, put that emergency spare on and have a new tire shipped 3-4 days ahead of you.

As for the worn tire blowout fears, just check the tread when you're airing up the tires. If you see threads, or a different color rubber in the center of the tire, it's time to change that tire. (Of course, that usually happens underneath the fender on the rear tire!)