Bloody hell, I joined MN 13 years ago!
Thanks for making me know that, Bubbles 
It's like a physical "pulling" feeling in my chest and tummy when I look out of the window and think about it. I have really realistic dreams that I am running, especially when stressed (so lots at the moment). I would give anything to be able to run again. It does feel like I grieve for it (I don't want to be insensitive to people grieving for people though as I know it's not the same)
That sounds like grief to me.
Of course it won't be the same as grieving a person, but I feel a physical yearning sometimes to be with my Dad, and I would give a lot just to have him back for a few hours.
It's not insensitive to have different kinds of grief, and talk about them.
Arguably the grief of losing the ability to run through is "worse" than the expected grief of losing a parent.
Although again, part of the pain for me is how much sooner I lost him than I had every right to expect.
Everyone eventually loses the ability to run, but losing it young is cruel and unfair.
I think it's important to talk about different kinds of grief.
If I never have to hear people minimising the grief of miscarriage or infertility again, it will be too soon.
Anyway, that went long. But at least it stopped me looking for speakeasies.