Right after I watched the trailer for the Amazon Studios film Brittany Runs a Marathon, I thought about the time I had forgotten to buy energy chews right before a 10-mile marathon training run. I walked to the grocery store in my running clothes—spandex leggings, a technical shirt with the name of my local running club on it, and New Balance 860s. I put two packets of chews on the conveyor and started to pull some money out of my belt pack when the cashier said, “These are really good.” I told him that it took me a while to...