I've done a lot of long rides fixed, even some fairly loaded (not quite as long!) ones. My long distance fixie has a 42x16 and I have gotten it over 60 kph *downhill*, for short distances. You can retail cover a lot of round, I you are willing to accept that as soon as it is not flat, you will be going slower (and sometimes a LOT) slower than if you had gears.
But as to what to get, the single most important thing for a bike you will spend all day on every day is that it's comfortable, and it fits. Weight starts to matter less when you're on a longer trip, especially if you're carrying your stuff anyway. If a fixed drive train is what you want then go for it, but buy a frame and the rest of the parts for what the actual use is, not for what drive train it has. In other words, a good fixed gear long distance bike has more in common with a good geared long distance bike than it does with a track bike.