When I run, I always think the same thing: That I’m conquering cancer. It’s like my mind plays on repeat as I put one foot in front of the other and pound the pavement. Maybe it’s the research clogging up my brain—the stuff that says five weekly hours of vigorous exercise helps prevent a recurrence of breast cancer. Maybe it’s the feeling of power I get from logging mile after mile, the knowledge that my body really is strong—if it were not, I wouldn’t be able to run. Maybe it’s both, or something else entirely. It doesn’t matter. What matters is that when I run, I believe I am leaving cancer far behind. I believe I am crossing the finish line, and cancer never will. Running is my preventative medicine. So far, it’s working.