I think most people can train fairly easily to be good runners, especially if it is just for a 3 mile run. I have **** knees and running was tough to get used to but I became pretty good at pacing myself for a couple hours. Running just takes a long time to build a base and some say you should do it for almost a year before taking to a competitive level. There's running "right" and running "wrong" and it may take some people that time to really get how to run "right" and not hurt themselves

I wouldn't say running is fun in the same way as cycling but runners high takes you to a different place and any need for any stimulation just disappears.