Saddle choice for long distance rides Every year I travel home to Nice, on my own, from a European capital with my bike ex. Paris, Salzburg, Naples, etc. 
Last year I started off from Madrid, and this year from Berlin; both of these trips took me nine days (~1500 kms).

I ride a Giant Defy Advanced bicycle equipped with disk brakes and Zipp 303 wheels.

My current saddle, ASTUTE Skylite, replaced the original Fizik saddle that came with the bicycle; I was told that most manufacturers equip their bikes with 'lesser' versions of saddles in order to trim the overall cost.

Previously I rode a TREK fitted with a Selle Italia saddle.

Sizewise I'm 1,72m and 68kgs

I wear an Assos T.Cento bib designed for long distances (purchased 2015) and apply copious amounts of Assos chamois cream

In preparation, during the year, I cycle once a week 60-80k with at least a 500m climb

My main difficulty during these trips is my back side

The sit-bone area becomes swollen with a sore on each side which gets understandably quite painful; Compeed and Doliprane/Neurofen are my only recourse

Each evening I wash my clothes with shampoo in my hotel room

Having adopted a vegan diet I have also chosen to avoid leather based products.

I'm still searching for THE saddle that will allow me to pursue such trips without the suffering.

I'm currently considering a Brooks C15 (or C13 145) saddle

Should I accept that it is the mileage and duration of these trips, or even lack of preparation, that are to blame for my plight and will remain thus whatever saddle I may choose ?

cheers
Get your sit bones measured.  

Bikes are made in different sizes, handlebars in different widths, cranks and stems in different lengths, seat tubes are adjustable.  Why all this variation?  Because people are built differently.  So why just choose a saddle that "should be good" and go?  Get your sit bones measured, then buy a saddle from a company that makes different width saddles to accommodate different riders anatomy accordingly.