My dream is to be able to run again. Please. Let me run a 10k at least once more in my life. To feel that stillness and freedom and calm that sets in when the brain start going to hibernation after about 7km.
Check the ElliptiGo. It gives you a similar feeling without the impact on the knees or hip. I know someone who was an avid long life runner. He substituted running for using the ElliptiGo for years and seemed very happy with it. Surprisingly, he was able to run again. I also have the ElliptiGo, but I don't have knee problems, I started using it because at one time I was having neck pain and couldn't run, bike or swim.
Hope you see your dream realized. But know that that stillness is achievable through other activities as well. Most directly and deeply, through a meditation practice which is geared towards reaching those deep meditation states (called Jhanas in the Pali canon). My favorite guide on that particular path is Leigh Brasington.
I broke my ankle nearly two years ago. I've had three surgeries already and will be getting a total ankle replacement in about a month. Even with that, I will never run again.
Sometimes in a dream, I'll start running. I'll notice how magically effortless it feels. How wonderful to be able to run again. Then a little voice in the back of my head reminds me that this can't be real. It wakes me up every time.
It was a rough day when I opened Strava to log one of my physical therapy walks and realized that if I scroll down a bit, I can find a record of the longest run I will ever do.
I'm mostly at peace with it now. I'm grateful that at least I was into running for a while before I lost it, so at least I don't regret never having done it. And I never really enjoyed it then anyway. I just did it for health reasons and the sense of accomplishment.
I'm sorry for your suffering. I know what this longing feels like.
That would be quiet something to feel that again.