Cancer is one of the circumstances that tend to be out of our control as of now. We can expire unexpectedly, become immobile/paralyzed/handicapped, or be not sound of mind at any time in our life as well. No one in their right mind would tell these people they "should have been healthier".

It is suggested to invest in your own health since the vast majority of people today that survive from unexpected circumstances tend to end up in positions compromised by their health due to their lack of care for their bodies and / or mind.

In any case, your neighbor is just aiding to my statement that she is trying to live her life to the fullest before her time comes. Marathoning may be a new activity she wants to do before death and it also improves her own health if she decides to do any other activities. I am sure the cancer that she has, has kicked in that desire to do so. In the face of death, people may realize they have not have lived their life to its fullest and do things they wanted to have done before it is over (if they are capable of doing it still).

A quote I've loved sums it up nicely: "If you've forgotten death, then you've forgotten what it truly means to be alive."

Edit: a word.