Age 48 and I am definitely in the category of running to be a better runner and working through all the problems that come up. I currently average about 60 mpw with 2 faster workouts each week.
I have thought for a long time that breaking 5 in the mile at age 50 is a good goal, so I will likely try that. Aside from that, I have no goals other than to keep doing what I'm doing as long as I can. If that means doing more cross training (e.g. elliptical, stationary bike, ElliptiGo) as I get older, then I am prepared to do that. Like another poster, I am semi-retired so I have plenty of time to train and work on rehab/prehab, which for me is a 6-7 day per week endeavor.
I think the key is to love the process the more than the results. If you enjoy the process, then none of this will feel like a burden. Another poster asked why risk injury at this age, again if you love what you're doing, you can easily tolerate the highs and lows that come with it.