I'm sorry to hear that your walking partner does not understand that you must walk slowly due to your recent round of chemo and radiation. Here's what to say to your walking partner:
"I know you want to get more exercise, so go ahead and walk fast ahead of me. Then turn around and walk back to me. You keep repeating this forward and backward loop, and I'll keep going at my own pace toward you."
Good luck with your recovery and keep walking. The speed doesn't matter. It's the consistency of practice that is important.
Dr. Joan's Harmony Key: Figure a way that works for you AND your partner.