I didn't run at all until I was 30 (mid 30s now), progressed quickly into road marathons with a 2:44 time this past December.
I also completed my first mountain 100k ultra last year at high altitude and caught the bug. I'm fully transitioning over and building up to a 100 miler in July (Montana), with a 100k in 3 weeks (Colorado), and a 50 miler in June (Provo). To top it off I signed up for the Rut 50k in Big Sky in September because it looks like a good time, although the timing sucks, it is appropriately named since it's right in time for the elk rut.
I just like pushing myself to see what I'm capable of. This past weekend I had a 5 hour run on Saturday (ended up doing 29 miles in the mountains) and 4 hours on Sunday (another 24 miles). It definitely builds leg strength and I've never felt better. People keep saying running is bad for your knees, but I tend to hear that from people who don't run.
I'm sure as I get older I'll transition more into cycling or something with less impact, which I do also enjoy. I'm planning to go out of this world with no tread left on my metaphorical tires.