Hello Jenny,
THe GGW was our second long distance walk, and in Holland we are not accustomed to hill walking (flat as you might know) so we planned to do the walk in 6 days.
The start is rather easy going and a bit dull along the canal.
Our main goal was to keep enough energy to enjoy everything so 6 days were fine and comfortable.
We choose hostels and where not available a B&B. I can recommend the Loch Ness Hostel it is very nice situated and has a real good atmosphere!
For a good meal I recommend the Fiddlers Inn (also B&B!). After Drumnadrochit the GGW is getting steeper.
For us the last track to Inverness was beautifull but heavy going, especially when after reaching the end we had to walk quite a distance to the hostel! It is desolate but very beautiful!
We are 50, and if you are younger it might be easier.
Enjoy your trip
