Assuming that this is your first RTW segment, the best part of the flight will be arriving in MAD and accessing the Velazquez lounge in Terminal 1. It is not a "grueling" flight by any means and most certainly beats taking a red-eye on any AA 757 aircraft.

There are a few ways to get reasonably comfortable in the IB seat. My preference is the inflatable neck pillow, which I find helpful to use with some of the carriers biz class seats.