I'm not entirely convinced about the general slowing-down advice when you have a lot of miles to cover. Making good progress while it's easy (flat and/or with a tailwind) is good for morale, especially if riding alone.
@User doesn't mention whether there will be anyone riding with him. Moving as fast as you can without really pushing yourself, when your legs are fresh, also leaves you time when it matters - to have proper breaks with real food. Ideally, If I were doing a 100+ miler with the tougher stuff in the second half, I'd aim to have porridge or muesli for breakfast, get on the road at 6:30, and have about 40 miles behind me before stopping for a second breakfast of something like a bacon roll or beans on toast, with plenty of tea - the key thing being to stop before you feel really tired or hungry, but to press on another few miles if you're feeling lively. There's no need to rush the stops - psychologically I like to give myself an hour for a proper meal stop, but actually leave with 10 mins or so to spare, so that I'm already a couple of miles down the road when I "should" be leaving. An early start means that you can have a well-fed morning and the timing will still work for a proper lunch stop according to duff British food-serving conventions. Light but substantial lunch is the answer - something like a jacket spud being ideal. If you've got 70+ miles behind you by lunch, you're laughing. Then you can slow down when you really need to - for the tough, hilly stuff - and just accept the inevitable plummeting of your average speed in the last third of the ride, knowing you've earned yourself a nice cushion of afternoon time. I am not one for attacking long hills, but I like to milk the downhills for everything they are worth. You reach your destination, have a nice bath or shower, a decent meal and as much celebratory beer as you can handle without courting a hangover, go to bed earlyish, and repeat...