Don't worry!
You'll have an absolutely fabulous time!
One of you needs to carry the details and the other the maps. Stop your horses from time to time (they will just eat) and read the directions slowly and check that you can follow them on the map as well - that will help you to look out for landmarks.
If you have no previous experience of map reading, ask Matt to give you some pointers before you leave HQ - it's his passion - or go on the OS website and find their tutorials. You'll soon get the hang of it, but be prepared for some disagreements along the way as to the correct route - you can always make up later over a drink and meal. There is no need to rush - you will have plenty of time - just enjoy the whole thing.
The Begwns will be lovely at his time of year (if it stops raining long enough for you to appreciate them). Where are you staying?