With the rapidly increasing popularity and choice of endurance bikes, the question is, what is the best mix of ingredients for making a long rides enjoyable and less tiring.
On one end of the spectrum are comfortable, a little heavier, and not very aerodynamic endurance bikes, such as Specialized Roubaix SL4, Bianchi Infinito CV, Trek Domane, and others. They are comfortable to ride, handle well, climb fairly well, and make a longer ride less painful. However, the two areas where they are less fun are shorter rides where you want to go fast and very long rides -- 100 miles or longer with lots of climbing and head winds.
The relaxed geometry and weight get in the way for short and fast rides. For example, I used to commute 20 miles to work. I had noticed a pretty big difference between Specialized Roubaix SL4 and a more aggressive road bike, where the former would make me noticeably more tired by the time I got to work.
As far as long rides are concerned, I feel like I get more tired with the comfortable ride and relaxed geometry, which turns into a longer ride and more effort than on a slightly sportier road bike that is not as comfortable, but is faster and takes less effort to get going and keeping the speed.
So far, Pinarello Gan K seems to have the winning balance between somewhat relaxed and aero geometry. We will see how it holds during lonver rides.