Think of your GPS as an 'electronic map', though you have to 'draw' the map yourself first of all. The way that we do this is:
1) Plan the route with a map as normal
2) Decide upon 'waypoints' that you would like to use on the map. These would be (for example) car park, path junction, river bridge, pub, etc. You can have as many of these as you like.
3) Work out the grid-references of each of these waypoints from the map. The standard is to use 6-fig with the map, but the GPS will hold a 10-fig (which should be accurate to 1 metre)
4) Program these waypoints onto the GPS
5) Once you have them programmed, you can navigate from the 1st to the 2nd to the 3rd waypoint until you have reached the end of the walk. Suppose that you park the car at waypoint 1. Select waypoint 2 on the GPS. The GPS will then tell you which direction to take, the distance to go, and an ETA of your arrival time based upon your walking speed. It also telsl you how far you have walked in total etc.
Hope this makes sense and is not stating the obvious. Let me know if you need help seting up the GPS and putting in waypoints.