Why Map My Run is rubbish
Map My Run and similar mapping sites are extremely useful tools to plan a route,but the distance given will be approximate.
To illustrate this have a look at the picture below,taken from the York Marathon. A measurer will always try to measure what is known as the "Shortest possible route". This means keeping tight on the bends and cutting corners as much as possible,while remaining on the course.(because this is what runners will do!)
The yellow route is the route the measurer will take,the red being what Map My Run has automatically calculated.This means that the stated distance for any particular race will usually appear too high on MMR,or if you are planning a run and get it spot on,it is probably still a little short.