You can destroy a couple of links by breaking and twisting a chain or something. Just cut your chiain and put it back together with your chain tool. You'll have to be careful about cross-chaining (Big/Big), but otherwise it should be rideable. Apparently if you blow out a derailleur, you can bypass it and choose a gear that will get you to most of where you have to go until you can get to town.

I think the Surly LHT has horizontal dropouts, which allows a single-speed configuration in a pinch.

Technically you're supposed to splice 9s and better chains using special chain pins (Shimano brand, use whatever brand you need) So far, I have had good luck just pushing out pins (never past 90%), and pushing them back in with my 9s chains, but an extra pin or two would be feather light, and could help in a pinch. Or, maybe carry a spare quick link.

As far as chain wear. I used to be be pretty relaxed about chain wear. But, as the chains wear, they "stretch". Essentially the bushings wear so they are no longer tight. Anyway, when you get about 1/16" stretch per foot, then it is time to replace the WHOLE chain. Not just a few links. Go much over that and you risk damaging your cassette and/or chainrings.

Chain life apparently varies considerably. Under pretty harsh conditions, maybe 1000 to 2000 miles per chain. Under ideal conditions, significantly more. Leave the chain on too long, and you blow out your cassette with the first chain (I think the new cassettes wear faster than the old freewheels).

So, 5000 miles, with a generally well lubed chain, mostly dry, mostly pavement (dirt?). You'd probably be pushing it to last for the trip. 2 chains would probably do it. Really bad, wet, dirty, muddy conditions, even up to 5 chains

As mentioned, for the kind of mileage you're wanting to do, start with good new consumables (chains, tires, etc).