In my experience breaking spokes is not correlated with number of kilometres ridden, it has more to do with keeping the wheel in good repair (not warped, even tension on the spokes) and avoiding impacts.
I would inspect the wheel, or have it checked at a shop, to make sure it's in good condition, and given that I would ride it without worry. I would, however, bring a few spare spokes with me on a long tour, just in case. (They don't weigh much.) If you're checking the wheels yourself you should look for traces of corrosion at the spoke nipple (dissimilar metals in contact, so the most likely place for it to start) and check the length of each spoke for nicks or deep scratches, as from scraping against a rock or the chain getting caught on it. Assuming no apparent damage and even tension, you should be good.
I wouldn't be overly concerned about a single failed spoke, especially if it's been replaced for a few thousand kilometres without further incident. That single failure could have been a lone substandard spoke or, more likely, some localized damage like hitting a rock, getting hung up on something when leaned (or leaned against), accidentally dropping a tool on it, etc.
Note that there is no intrinsic reason for spokes to fail with use. Properly and evenly tensioned there should not be enough flex in the spokes to cause metal fatigue. (If you were fatiguing the spokes they would wear out in a lot less than 6 000 km.) If they are free of corrosion (the other probable cause of failure with age) then they should last for decades. Embrittlement should be extremely rare given the conditions under which bikes are stored and used.
I've had a couple of touring bikes on which I have put tens of thousands of kilometres. I've gone through a few wheelsets; I had to replace one after the rear wheel failed due to wear on the brake track. Those wheels had approximately 40 000 km on them, and the front wheel had never broken a spoke. (I had another wheel fail after about 25 000 km when the hub flange cracked; the spokes were fine.)
I have, on the other hand, broken a lot of spokes. Most of those can be directly attributed to abusing the wheels, for example by riding off a kerb with loaded panniers. (Or rather, doing that many times.)
I don't want to sound like a certain other user, but if you're loaded touring a high spoke count is your friend. It both reduces the likelihood of breaking a spoke in the first place and also makes it a lot easier to cope with a broken spoke if it happens.