Your all important Walker number, of course! But also its worth checking out the weather forecast the day before and making sure you're dressed for it.
Personally, we would recommend sunscreen, midge repellant and a light waterproof plus if you're bringing a dog, your other doggie accoutrement including poo bags of course!
You will also need to either bring a reusable water bottle or use a Walking for Wag bottle you’ll be given on the day