We all have different issues, but for what it's worth, my dad ran well into his eighties. He was an avid tennis player his whole adult life and developed some knee problems from that sport. To stay in shape for tennis, he ran - way before running was cool - way before the running shoe industry became such a thing as it is now. At the time, the medical practice was removal of the cartilages and therapy. He was about fifty when this happened, but he was determined to keep running. He developed his leg strength with a weighted boot of his own creation. (The man could rig anything up with duct-tape.) I don't even think running form was on his mind, but I also remember that he did change the way he ran, landing more on a bent knee, taking shorter strides. He completed a marathon with those doctored knees.

So anyway, part of it is "what you have", as SolaceEasy said; and part of it is that what you have might be able to keep working - with modifications.

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I actually started running in my 50’s and for 3-4 years it was great. Then the maker of my best shoes decided to change their Cadillac to Cruz - and my running suffered greatly as did I. I have found the proper shoes by a different maker and that made a huge difference. However - I still couldn’t sustain 2/3s of the distance I previously ran on any given night and running in the cold which had been a fave was right out. In talking to friends I decided to try inserts. AMAZING! In my first run with them I sustained my mileage at what it had been but without stops and at a faster pace (I would not consider myself fast by any standard but I was faster than pre-inserts.) The next day I added a half mile. The next day I added another half mile increasing my mileage by a mile in two days. While my legs felt worked they were not sore. No hip, knee, ankle, Achilles or back pain. I took comfort in knowing my cardio was there all along - it was other issues interfering in my ability to make sustained runs. 

I don’t know your issues or thoughts but you may want to check out your feet. They change with age and miles. They can be the last thing considered too. 

I don’t know if this helps but I wish you all the best and a resolution to your issues. Nothing beats being able to get back out there and enjoy the mind clearing freedom of a good run. Take care!

I'm 61 now. I ran consistently from age 28 to 35. Took some time off to raise kids, and then started running again in my mid-40s, and started running 50 miler ultras at 50. Backed off to avoid injuries but still running fairly consistently with many months off here and there due to injuries.  While I was never fast I did notice that I did progressively go slower over the years.  It's inevitable.

Now I just run 15-25 miles per week at a very slow pace, using the Maffetone method just to keep my aerobic base going.  I recommend this approach.  You just go very slow but go long, >1 to 2 hours to keep building you base w/o injury.  Since adopting Maffetone I have been injury free for several years and I feel great and I love my runs.  I also love that strenous hikes count as maffetone training.

There is no need to run fast.  Just stay in your aerobic zone. Google it up.

Aging sucks.  I'm a mountain dirt trail runner at 61 still...... and I'm going to keep running till my knees give out....literally!! I'm grateful that I can still do what I do at over 7000+ ft in elevation, on technical mountain trails and away from most people in the solitude of nature. I've been running/staying active for most of my life and I truly believe it's one of the reasons I'm still doing what I do. I ran before and after marriage, having kids, finishing my degrees while working FT, and moving to different states and changing jobs, life's crises, etc.. However, as a person of color, my main hesitation is running in places (when I travel) that I 'know' I'm not welcome or avoided but I still show up and finish. 

I consider myself a 'social' person and greet other runners but it seems like I make them 'feel' awkward ......... I don't take it personally, it's their problem. But it sure would make for a more 'fun' event at the end. I love the comaraderie of post-run events. Having said that, I don't miss much about racing because I don't think I have lost much between pre-COVID and present races that have COVID rules. I am not comfortable running races even now ......probably till next year (2022). So with aging (in my experience), I keep to myself, run against myself, run to enjoy the event, and as DharmaDude stated "no need to run fast."  I'm always in training and enjoy running 50ks......however I love hiking and camping as well. Just trying to keep balance in my activities so I don't get depressed on what sucks with running as you age.  

Take it easy, find gratitude, embrace the life changes, and as the saying goes, "it's suppose to be fun."