Might be worth considering a Garmin Oregon.
I bought one earlier this year and used it to guide me along the MTB C2C.
It comes with a complete set of the UK OS 1:50,000 maps, which are pretty good for off-road cycling.
I drew the route as a track on MM, one track for each day, and transferred them across. Each day I opened the appropriate route, zoomed into the map and followed the track.
It uses AA batteries and the Lithium ones lasted a day and a half (around 18 hours) in the energy saving mode.
Had absolutely no problems with it, you can zoom in and out to see as much as you want and it is so much easier than carrying a map.
It aligns the screen to the direction you're facing, shows you as an arrow near the bottom of the screen, shows the OS map and your route drawn on top.
I think it's a fantastic piece of kit and it must have saved at least an hour a day pulling out the map, confirming where I was and then working out where I should go.
If you want an electronic map, then this is ideal.