Today, I finished work around noon, laced up my sneakers and threw on my husband’s fleece pullover, and walked right out the door, down the driveway, and onto the street. For miles.

Well, two miles.

But it’s two more miles than I walked yesterday, or the day before, or the last two and a half weeks, actually. Since I had surgery in early February, the most walking I have done has been through the hospital hallways for a follow-up appointment and up and down the aisles at Costco on a recent shopping trip.

It’s been just a little more than two weeks, but it’s time to start walking again, and hopefully running soon. It’s time, because this body needs the exercise. I have a heart that needs to pump a little harder and lungs that need to expand with air. I have blood that needs more oxygen, and a brain that needs to be cleared.

Not to mention, every one of my created little cells need the benefits of exercise, the benefits that counteract aging and stress, just like they need the vegetables and fruits and grains I feed them and the sleep I get each night. That’s how I think about my health lately. Since cancer got written into the story of my life, I often feel betrayed on the cellular level.

The only thing I know to do? Offer life to my cells so they can grow and flourish and bring life to the rest of me.

It’s ironic, really. Because bringing life to cells is just half the plan for fighting cancer. The other half is bringing death to cells—death to a small area where we think cancer might be lingering. After a very successful surgery in which no further evidence of disease has been detected, tomorrow I start radiation therapy. Just in case.

Radiation also works at—you guessed it—the cellular level. By aiming radioactive beams to small areas of the body, the genetic material within cancer cells is damaged, limiting their ability to reproduce. So, those cells die. That’s the good news.

The bad news is that all of the healthy cells, the ones that I am caring for through exercise and diet and rest, they die from radiation, too. The same therapy that heals me also kills me, or at least little parts of me. The hope it that when the killing is done, and the exercising and the eating and the sleeping continue, eventually, the good cells will return. And the bad cells will be gone forever.

I have made hundreds of big decisions in my life, decisions about where to live, what job to take, which church to attend, and recently, whether to get married. I have made thousands more medium-sized decisions about what books to read and which restaurants to eat at and whether to go shopping or save my money.

But when it comes down to it, it’s really the millions of little things that happen at the cellular level—some of it I do to myself (like going to bed on time and exercising), some of it is done to me (like chemical exposure and genetic anomalies)—that brings life or death to a body.