Find a map of the entire area that the path crosses. Mark the start and finish points of the path.
Tie a knot at one end of a string that is about the same length as a diagonal across the map. Place the knot on the mark at the start of the path.
Stretch the string along the path, holding it tight with a finger, until you reach the first turn. Hold the string where it is on the map at this point, and stretch the remaining string along the next part of the path. Continue in this manner until you reach the finish point of the path. Tie a second knot on the string at this point.
Stretch the string alongside the ruler's marked side, with the first knot at the zero mark. Measure the length of the string between the knots, either in inches or centimeters.
Read the scale on the map. It will show the actual distance represented by an inch or centimeter on the map. Multiply the string's length by the scale's value, as it appears on the map. (For example, if the scale says one inch on the map is equal to 10 miles of the real distance, multiply 5,280 -- the number of inches in a mile -- by 10 to get the inch-to-inch ratio, then multiply the resulting number by the string's length.) The final product is the path's distance, which you can convert to any measurement of your choice.