In the spirit of starting to run when over 50, I thought I'd share my own experience: I'm 57 now, started running at 52, all on my own - no trainers, coaches or running group. I just thought that running would be a good thing to do from a cardio-vascular perspective... and so I charged into it. I struggled mightily, but two years later I ran my first marathon. And following that first marathon I had so much over-exuberance that I had this insane idea that if I could run one marathon, why not two in one year? So I started training and running and just went crazy about getting conditioned - and wound up repeatedly hurting myself. But instead of letting myself fully heal, as soon as most of the pain from what ever it was that I injured was gone, I'd be back on the road or trail again - and sure enough, I'd get injured again. I literally ricocheted from one injury to another. My net result was that I didn't run any marathons that year (2012) and the two HM's that I did run were horrible and not fun at all. So I vowed to manage my "exuberance" and as a result 2013 was much better running year.
The points that shipo made are right on... don't over-do it. Watch your pace and your growth. It is way more fun to run when nothing hurts!!
-bill