If anyone needs any inspiration for being over 50, and starting running, do some research on actor/marathoner John Keston, who at age 55 started running from scratch, and went on to set numerous age group running records. Now he's in his 80's, and still runs.
Myself, I'm 50, and have been running every other day, typically 6.5 miles. I started about 2.5 years ago, and at first I could barely do 3/4 mile on a treadmill. Now, I am at that point where I can do a long run of 15 to 20 miles, and I've got my first marathon scheduled.
What worked for me is having backup routines, like cross training and bicycling, which I fell back on when I had aches and pains from running. I always try and keep those routines strenous by getting my heart rate up to target, or about 90 percent of max. Also what helped is joining a running club with recurring running events throughout the year. They have online databases of your times, as well as those of the people you want to beat. The future races, and my past times motivated me to keep going, and improve.
It is so worthwhile though, just for the endorphines, higher self esteem and better moods. Good luck to all!