The best tyre for longevity is the Marathon XR, not the Marathon or Marathon plus. The rubber is different meaning you get even more kms from a tyre and the newish Evo range are supremely puncture resistant. The Marathon plus is not really intended as a touring tyre, more as a commuting tyre. The plus won't last as long as the XR.
Expect around 20,000km from a set. Only thing that has punctured mine are viscious african thorns, but only twice. A miracle really.
They're expensive, but cost per km must be better than any other touring tyre.