Sketch the lawn and draw the general tracks along which you mow.
Measure the segments on the actual lawn. If the lawn is in a basic shape like a square, you might have only one segment or track to measure. If you have an irregularly shaped lawn that requires you to switch tracks and mow varying lengths, measure one of each track.
Multiply the measured tracks by the number of tracks. If you have one track of 10 feet, for example, in a square lawn that requires you to turn around and repeat that similar track 20 times to cover the lawn, multiply 10 feet by 20 to get 200 feet.
Add together all the values you get for the entire mowed area.
Divide the total number of feet by 5,280 to get the number of miles. Two hundred feet, for example, is 0.037878 miles. Alternately, you can multiply the number of feet by 0.00018939 to get the value in miles.