Exercise during chemotherapy, radiotherapy, or hormone therapy is highly recommended and perfectly safe.
Moderate intensity exercise won't make your fatigue any worse.
In fact, by exercising regularly you'll be able to deal with the fatigue much better than if you just allow it to take hold.
This programme aims to maintain as much fitness and strength as possible, but each day is taken as it comes in terms of what is achievable.
Above all, you'll be fighting back, and by maintaining your strength and fitness, you'll be doing all you can to assist the medical treatments.