I agree Kneeophyte (great login name!), I would also check out the buckling thing, too... when you get back. That's the kind of thing my pre-TKR knee would do, and it was known to drop me in my tracks one too many times! I'm thinking about how the knee is constructed, so buckling could theoretically mean that the quadriceps somehow wasn't holding on to what it's supposed to hold on to...
But, as for the distance you're able to do hiking or walking, that's about where I think normal might be.... I'm just past 11 weeks and did 1.33 miles today, but came home and iced, used my herb/CBD salve, then elevated my knee this afternoon for a bunch of time (since I had to get some computer work done anyway.
Brian, I don't know if you have and or use hiking poles? Back in the day, I taught Nordic Walking using very lightweight poles that had hand gloves that fit your hands well and you used the poles to help propel yourself forward as they push off behind you as they move backwards. I've begun using them again now to get better results from my walks. Their use aids posture straightness, helps condition the upper body some because you do use your triceps to push your arms forward, plus the posture aspect is proving very helpful. I wonder if you used hiking poles if that would help your upper body take some of the load and give your knees a bit more ease? I bet you could find a pair in any REI or outdoor-oriented store in a town you pass through?