I wish you luck on your journey! It’s a delicate balance of exercise and rest. Being fit will definitely help you endure the rigors of treatment but, you have to guard against too much fatigue too so that your immune system can do it’s job. And I was usually wrecked by the 3rd day of chemo (I did 3 days on, then 2 weeks off for 5 months) so exercise was out of the question for me for a few days after each “on” week ended.
Regardless, being on the bike always helped my mentality too!