I am a 53 year old ultra runner. I have completed a couple of 100 mile events along the South Downs, a 130 mile event along the Liverpool to Leeds Canal and in June 2017 a 250 mile event around the UK canals.
I am not the fastest ultra runner but I do train 5 times a week with long runs back to back (of 3 to 5 hours each) on Saturday and Sunday at peak training. At the end of last year I was training for a 100 mile event in America for Feb 2018 and was diagnosed with breast cancer (including 3 lymph nodes) at the end of Nov 2017. I had to cancel my entry and curtail my running. However, being stubborn and being told that exercise through chemo was good for you I continued to run 5 times a week. I even managed a 60 mile week during my 2nd cycle. I only stopped my running completely whilst in hospital and recovering from a bad reaction to one of the cycles but I still walked at that time. I had an aim of entering a 100 mile event this October but as fate would have it (well actually it was my own fault for over doing it as I reached the end of treatment!!) I got an over use injury which is now forcing me to stop running until it heals. Ironic really but I do feel having a focus through my chemo helped get me through treatment.
They are my personal experiences and obviously not advocated for all but the difference I felt after exercise, however crap I felt beforehand, was always positive and I feel it made a significant difference to how I coped through my treatment.