There's a good thread already on wear of this model of tire here: When does a Marathon Plus tire (not) need replacing?.
I'll repeat a shorter version of my answer from there:
- The Schwalbe Smart Guard is an extra layer of rubber between the road side and tube side of the tire, and depending on tire model it is one of a few non-black colors. Check if you can see the colored middle layer anywhere, if so , replace the tire since you've broken through to the middle layer and the road side of the tire could start delaminating.
- If you want to keep using your tire but are getting lots of flats, check for small embedded puncturing agents e.g. small pieces of glass or radial belt wires from transport truck tire blowouts.
- If the rear is very worn and front looks good, resist the temptation to swap them between front and rear wheels (AKA rotating the tires). A worn tire is a greater candidate for blowouts, which are more likely to cause injury when they happen on the front wheel. If you don't want to replace both tires, then chuck the worn rear tire, move the partially worn front tire to the rear, and put a nice new tire on the front. More info on this from Sheldon Brown here.
FYI, I finished an 8,200km tour on these tires with no punctures, and a decent but not complete level of wear on the rear. I would not have started another long tour with the same tires.
IMO, your tire probably has 5,000km left in it, but you should suck it up and buy a new tire before your next tour so you're less likely to need to replace mid-tour with the challenges that come along with that (Where to buy replacement? Do you need to get them shipped to a hotel? Etc.). When you're finished your second tour, you'll probably have 4 Marathon tires that all have a decent bit of life left in them and you can run them into the ground commuting around town or Randoneurring, situations in which it's not a big deal to have a tire wear out (just replace it at home at the end of the day). That way, you've got plenty of tread for your upcoming tour, and no tires are going to waste long term.