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.
Some emerging concerns I'm aware of for really serious runners: heart problems due to thickened heart wall, skin cancer (just due to being out in the sun so much, sweating off sunscreen). Potential causes for concern: lots of cortisol production from hard aerobic exercise, inflammation.
This post is a bit of an experiment; Most of the time, Discussion post lay out an idea and this idea then get commented upon. This post, on the other hand, will be purely about discussion on a topic. If this works out well, I'll might post more of these in the future.
On to the meat of this post:
I got this idea from a reddit post on /r/LessWrong.
To quote:
So have at it.
I only ask for one thing and that is to hold off on proposing solutions for 24 hours before giving suggestions for actual sports. In the first 24 hours, please discuss what makes current sports unappealing to rationalists and what aspects a sports designed for aspiring rationalists should have.
Edit: The 24-hour window has closed and solutions and suggestions can now be given.