Hi dieterreuther, and welcome to a club you never dreamed of joining . . . I'm about 9 years into my PN journey. I have absolutely no medical teaching; I'm simply a patient, so take this or leave it. My feeling is to do as much exercise as you want and can while you can. I think the stronger a person is when they enter the more debilitating effects of this process, the slower the effects will be. I have nothing to back this up other than my experience. Be good to your feet, make certain you are with a medical provider you trust and have confidence in, and enjoy what gives you pleasure. (I only ran one marathon, many years ago, the Seaside Marathon, in Seaside, OR.)