To determine your training route, a bicycle (or hiking) GPS alone is not enough.
Have a map of
the area you are going to travel through: GPS mapping
allows you to know all the details of the roads and paths you
Find the best Garmin maps
on the market: Memory Maps, Garmin Topo France whole, South-East, South-West...
The hex in its slightest irregularities of terrain will no longer hold any secrets for you!