To express the distance of a marathon run of 26 mi 385 yd in meters, we need to convert both miles and yards to meters.

1 mile = 1609.34 meters

1 yard = 0.9144 meters

So, 26 miles = 26 * 1609.34 meters = 42243.84 meters

And, 385 yards = 385 * 0.9144 meters = 352.032 meters

Therefore, the length of a marathon run of 26 mi 385 yd is 42243.84 meters + 352.032 meters = 42595.872 meters.

The length of a marathon run of 26 mi 385 yd is approximately 42595.872 meters.