Taking your walking or running program from the outdoors track or trail to an indoor treadmill can require making some adjustments, including changing your usual way of measuring distance. Treadmills tend to calculate distance in miles per hour, or mph; 400 yards is .23 miles, or just short of a quarter of a mile. To compare, the length of a standard running track, as set by the International Association of Athletics Federations, is 400 meters, which is equivalent to a quarter of a mile. Therefore, 400 yards on a treadmill is just about the same distance as a lap around the track.
Limiting your treadmill session to 400 yards, or a quarter of a mile, will likely not allow you to meet the minimum physical activity requirements set by the Centers for Disease Control and Prevention. The CDC recommends that healthy adults engage in at least two and a half hours of moderate to intense cardio activity per week. However, the first 400 yards can be used as a warm-up period, where you run or walk at a slower pace, allowing your blood to flow through your body and carry oxygen to your muscles as you prepare for more intense activity. Always increase your pace gradually.