Hi Poohbrains, and welcome!
I'm afraid all of us have to slow down as we get older. I know I have. (Not that I was much of a runner when I was younger.)
With VVs, as soon as their details start to show them as "elderly" they slow down. With runners the change is particularly startling. You can put off the arrival of "elderliness" by getting L2 or L3 medicine, but even at the top level they will eventually become old.

I don't think anyone here has been able to breed runners to produce more runners. They just seem to appear at random.