This is something I've always pondered. Does it really matter so long as each century is an out and back loop where you retrace the first 50 miles in the opposite direction? Total elevation gain will be zero no matter how hilly or flat the route. On the hilly rides you'll make up time lost going uphill on the downhill. On a flat course you have a constant pace and also travel further in terms of nautical miles. So could each century actually be equal in effort?

This gets thrown out the window if its a group ride where it is beneficial to draft in the flat fast sections.