Hi Emmy - I did the 50k this year, and friends did the 80k. Very well marked and amazed there was so much woodland close to the centre of Paris, quite hilly and this year was in the blazing hot weekend we had in March so felt quite tough. The last 10K or so is along the Seine which isn't the prettiest, but finishing at the Eiffel Tower was good, and for the 80Kers they got to run to the first floor of the tower, tough stuff. The trails are distinct, narrow in places so you are in single file, only a couple of aid stations so you need to be self supported. I was very good at fuelling up afterwards though! One tip, don't have any of their electrolyte drink, it tasted like an old people's home.