Hal ... what I was told by my neuro is that normally, when we exercise and stress the muscles, they rebound by growing stronger. With ALS, there is no rebound. When you stress an ALS-affected muscle, that stressed and weakened state becomes its new "normal."
However ... she said you can still exercise and strengthen the parts of the body that are not yet affected by ALS. Of course, in running, you are not building up specific muscle groups, you are building overall stamina. In my case, I continued with Pilates, using no resistance for upper body exercises, just range of motion, but continuing to use springs for resistance for leg work. Then my legs started to be affected, so it was all range of motion and stretching. Then I found even that to be too much exertion ... I was exhausted after a session ... so I finally quit.
Frankly ... I don't think anybody knows as much about ALS as we pretend to, including neuros. As a serious runner, you know your body best. I would use common sense ... I'm hoping they are wrong about your diagnosed too ... and not try to push yourself right now until you are more certain of the diagnosed. Don't take chances. They may be right. If they're wrong, you will be able to get back to your current fitness level.
ALS itself is a marathon, not a sprint.