I actually did not want to go too deep into discussing specific sports and wait for another 24 hours, but...

I never had actual problems with the knees myself - I'm neather heavy, nor run that much at all (100mile/week for 6 years is extremely impressive!), also I eat helthy and think my technique to be okay. But I am very young. My grandfather, who has been doing a lot of sports his whole life (to my knowledge he still rides his bicycle for 50 miles a day or something at age 80) had some knee problems and therefore changed from relatively serious marathon running (best time ~2:40) to swimming and bicycling. Of course these are just anecdotes that do not prove anything. I would be very interested in the current state of research on the matter.

For me the most important argument against long-distance running is that it seems to conflict with general fitness. After running my second marathon I pretty much sucked at everything else, even riding a bicycle...

Also, long-distance running takes a lot of time to practice, so now I changed to less than daily interval training, also supplemented by weight training.