In general, the difference is pretty clear. No, you wont get some magical added inertia that helps you complete the ride.
In the ways of advice, dont do it fixed. There is more reason not to, than for, if any reasons at all to do it fixed. You are going for long distance and coasting will be like heaven. The pro's for fixed riding are almost non-existant in terms of going for long rides, especially with down hills.