How to exercise safely while going through cancer treatment?

I used to be a personal trainer NASM certified and everything. I was diagnosed with CLL leukemia in 2019 and I really let myself go physically. The pandemic made Doordash a habit and I returned to a mostly sedentary job in 2021. Anyway, I'm trying to get back into decent condition. I was prescribed Calquence to treat the CLL and my current hemotologist/ oncologist has quartered the dosage I was taking. According to her, the drug is making my blood work read within normal range. I'm about 30-40 pounds overweight. I started exercising regularly in April. Anyways, I signed up for a fitness class in my local gym about three weeks ago. I figured that the class structure would help me with exercise. To make a long story short, I nearly blacked out during the class this morning. I'm scared right now because no one is really giving me practical guidance in this area. My primary doctor encouraged exercise and I pushed myself a little too hard. Has anyone else experienced anything similar to this? Exercising with cancer? Any takers?

Maybe you could start out walking in the park breathing in the fresh at your own pace. I have a senior pug and I thank God for her because she needs the exercise as well and she turned out to be a great motivator for me. When fatigue hits, I sit on a bench until it pass and continue walking. What ever you do, just one step at a time! Good luck to you.