Instead of worrying about battery life and probably running out, why not invest a measly few quid in the OS Landranger map of your area? This will show contingencies and bail-out routes as well as giving you an idea of overall scale and hills. Also it will show cafes and attractions along the route and give you a graphic picture of the terrain, which will give you a clearer understanding of the ride. Oh and you'll get years of use out of it and....NO BATTERIES....EVER!
I am a fan of the maps but I have never seen a cafe marked on any of the routes that I plotted using OS Landranger!
As for taking 4 hours at 10 mph to cover 66 miles ... I wonder if that is actually 66
kms? 66 kms is about 41 miles, so the numbers then would be correct.
I wouldn't expect a 41 mile ride to cause problems to someone who has managed a 19 mile ride without suffering too much, as long as the speed is low, the route is not hilly, the weather is ok, they eat and drink enough and are comfortable on the bike.