No reason not to ride long on fixed gear, if that's what you want to do. Obviously, there will be times when being over or under geared will be challenging. I ride fairly long (but not ultra long) on fixed very regularly and I enjoy it. Its not "practical" but who cares. Just work up to it so you know what you're getting into. Same as anything else, really.

I rode a 100 mile charity ride last weekend, and for 40 miles the headwind was truly brutal, being fixed. Ah, but life is good. And as somebody said, "it ain't about the bike".