This is a pretty tricky question because of the size changes the slime mold goes through during its life.
First, you need to calculate the number of body lengths for the marathon in humans. The average of height of a human is about 168.95 cm. A marathon is 42.195 km long. Dividing 42.195 km / 168.95 cm yields 24,974.84 body lengths. Keep in mind you have to watch what units you're using. If you convert 42.195 km into cm, you get 4,219,500 cm. Then you divide by 168.95 to get the body lengths.
Okay, so now we just need to multiply the size of the Dictyostelium discoideum by 24,974.84 to get the length of its marathon. If you are just looking at one cell, then it's length is around 16 µm (micrometers- millionths of a meter). 16 x 24,974.84 = 399,597.44 µm. It'd be better to put that into easily understood units, so you can convert it to cm by moving the decimal four places, giving you a marathon length for one cell of 39.96 cm!
However, since we're talking about a slime mold here, it has a "slug" form that is mobile, and more likely what you were thinking when asking this question. So we check the slug's size and it generally is about 3 mm long. Multiply 3 mm by 24,974.84 to get 74,924.52 mm. Again, we should convert that into a more easily understood unit. Moving the decimal three places gives us a marathon distance of 74.92 meters!
So to travel the same number of body lengths that a human does during a marathon, the slug would need to travel 74.92 m, or less than the length of a soccer field!