Several crossings under my belt, the most recent being in September last year.
You don't say whether you're doing a supported or unsupported crossing. If unsupported go light, especially if you are not used to walking any great distances with a backpack of any sort, backpack, headtorch, waterproofs, spare socks, food, liquid, first aid kit to include plenty of Compeed, if supported, take everything you think you might need and leave it in the support vehicle to be got at at the earliest opportunity if needs be.