You can use OS maps premium and use the national park view with snap to route, but I think os maps is very heavy on battery usage.
But as I cycle a lot I use strava to record my walk and bike rides, and it has a routes function with 99% of paths on, make your route before hand and then use the follow route option when you go walking, and strava is very light on battery usage also
Strava heat maps is the best map view ever, it shows you everywhere folk have been with the app running
, think of it veins and arterys of the country when you see it.