Probably until about 6 months before your knees finally give out. Have you considered eating cake on the beach instead? It will probably only shorten your life by around the same amount of time that you spend running.
You have no idea how much I've considered it. I walk through the bakery section while grocery shopping sometimes and fantasize about eating ALL of it. Even the cheap store brand stuff looks SO GOOD.
In terms of health and longevity, I suspect the vast majority of the benefit is in just not being sedentary and not eating all of the cake. Nearly all of the very old people I know have had the habit of taking long walks, but I cannot think of any who were athletes.
