Some runners don't have, or don't want to carry, devices to track their mileage. Some want to add calculations, or append data, beyond what apps allow. For whatever reason, people have developed a variety of spreadsheets to collect data and perform calculations pertaining to their running (not to mention their biking, walking, weightlifting, and other exercise).
The zip file offered here (see Files tab, above) incorporates a number of spreadsheets that I accumulated during a search online...