I think my friend has the same Magellan GPS - I rode the first 12.5 miles of Old Dominion and it said I went 8 miles. Rode 9 miles of trail at home (measured by odometer and scale maps) and it said I went 4.5 . I don't use it anymore.
I rode a "25" that was measured by GPS and had to push to finish in 6 hours.