You must have been pretty fit to tackle 100 km!
I also walk 5km (did this before and as much as possible during treatment) but split into two walks to fit into my day more easily. After treatment I saw a physio (really recommend this) who designed a short home program of Pilates with a couple of yoga stretches and deep breathing exercises which I do everyday. This has helped me to improve my core and has been good for my chest and arm on the surgery side. At the moment I also swim everyday but that will reduce back when life returns to normal again after the Christmas break.
Like Robyn says it helps with so many side effects of treatment. I only put on 3kg during 7 months of treatment and I have since lost that and a further 6kg despite being on tamoxifen. It is something you can control when it feels like BC has derailed your life! Best of luck with it all! Deanne x