Hi Tonniminniti01, I’m sorry to hear about your breast cancer diagnosis and it is great to hear that you have reached the point of recovery where you are running and training. It is not uncommon for breast cancer treatments to affect the heart and for you to perceive a reduction in performance. However, in most instances, training after these treatments are considered to be safe after testing to rule out any new cardiac issues. It would be good to do a Cardiac Echo Test before starting strenuous exercise. There may be other factors like low blood count that may also contribute to a reduction in performance. Regular exercise of appropriate intensity (this would have to be understood on a case to case basis) would help improve your performance as you recover. Watching for symptoms like pain, or tightness in your chest, shortness of breath, or palpitations is important to understand whether you need to seek further care with a physician.

I wish you the best with your recovery!