My knees hurt too, but celebrex pretty well takes care of the pain. : ) And I have health problems, type 2 diabetes the main concern. Running helps the diabetes and other health issues. Someone once asked me rhetorically to notice people who have knee replacements: NOT the athletes usually, but the ones who stay on the couch. I have to remember to stretch and do a variety of "weight-machines" up at the Y to help limber me up.
My dr is young, mid-30s or so, and runs marathons; rather than telling me to give up running, she tells me to keep at it and run more regularly. She did tell me to stick to a treadmill as much as possible to avoid injuries (I guess, falling down). So I watch TV either up at the Y or here at home (my hubby bought us [me] a nice treadmill, put it in back of the couch in our family room), and I watch Today or Morning Joe on msnbc while running.
But when knees do give out, biking is a good alternative; so is swimming, but my body in a swimsuit isn't a pretty sight. ; )
"adult-onset athlete" - I like that!!