The walkers take one hour (60 minutes)
The car journey takes 59 minutes
The car journey is at least N2 +2N minutes long (because of the stops)
If N=9, N2 + 2N > 59
If N=7 N2 + 2N > 59
If N=5 N2 + 2N > 59
Therefore N<7
If N=5
Travelling time is 24 mins (travelling at 50km/hr) + 35 minutes for stops
Distance = 24 x 50/60 km
= 20km

If N=3
Travelling time is 44 mins (travelling at 30km/hr) + 15 minutes for stops
Distance = 44 x 30/60 km
= 22km
But this would mean only one person is in the car and the question says "they" so there would need to be more than one. Also the appearance of the map would suggest that the road is relatively not that much longer and refuelling the car would need to be done by professionals! Etc.