It wasn’t only rain I was afraid of anymore; it was relapse.

My legs had been the first signal that something was wrong last year, too. Numbness had started in my feet, snuck up into my legs and my abdomen, and established itself in my hands. Typo after typo appeared on computer screens while I tried to type, and my handwriting became uncharacteristically sloppy. I couldn’t even tell if I was holding a pen unless I looked at my hands. At some point a doctor’s voice carefully explained that my scans revealed lesions in my brain and on my spine; my stomach dropped at the news and I was surprised I could still feel it at all.

Multiple sclerosis (MS) is a disease where the immune system attacks the central nervous system (the brain, spine, and optic nerves). I have a type called relapsing-remitting MS, which means that symptoms can come and go in flare-ups and remissions. The thing I fret about most, because it challenges my already shaky relationship with uncertainty, is the wide variation of MS disease progression. Anyone who has watched The West Wing can remember when President Jed Bartlett, who had relapsing-remitting MS, went from actively governing the country to being unable to move his arms and legs over the course of one airplane ride. Then President Bartlett was back on his feet with the help of a cane a few episodes later. The plight of real-life relapsing-remitting MS patients is similar: We don’t know what our bodies will do next. MS will either kind of change my life or really change my life, and there is no way to know which course will be mine.

Amazingly, thanks to many visits to neurologists, hours spent inside MRI machines, a couple of rounds of high-dose corticosteroids, and several months on an established treatment regimen, the symptoms of my multiple sclerosis are gone on most days. But today they were back, and more intense than ever. I imagined dozens of ways I could be humiliated. I could land a step poorly, fall flat on my face. I could push off my foot in the wrong way, fall flat on my face. I could slip on wet concrete, fall flat on my face. But, more importantly, what if the symptoms didn’t go away this time? I contemplated every alarming possibility. Was this a relapse? Was my disease getting worse? What if I stopped being able to run? To walk? Could I deteriorate to the point where I wouldn’t feel my legs again? What will eventually happen to my mind—my memory, my ability to think and communicate my thoughts to others?

I was terrified, close to tears, and there were 12 miles left to go.

I followed the race trail and walked on, tormented, obsessing about all of the terrible things that could happen to me today and in the future. I had some big worries: the neurons in my brain that could get damaged, the possible memory and mobility problems. But I also had smaller, pettier worries: what I’d think of myself if I didn’t finish this course, or what other people might think of a long finish time.