With a hint from "Methusalem" from the Wii Fit Forum (http://www.wiifitforum.org)
we now found a way to go from F7 to F8 using a quick one way jump shortcut.

The best route is now just following the numbers of the flags on the map!

We calculated the distances as before and came up with 7.57 miles (12.18 km).

We tested in an actual run and got 7.44 miles (11.97 km).
Next time we got 7.345 m.
and next time we got 7.286 m.
Considering that these are our first tries, we didn't used dog shortcuts, or took extra care in shortening the distances,
we're quite happy with the result and how close to the calculated distance we came.

What a day! a new bike, 4 stars, and a new record for us.