Did you Proffide the top and bottom one time before doing your 400 miles?
A guy 200lbs will break in that saddle with in 400 miles a 160 lb. guy will take twice that.
You also need to back off the tension bolt one possibly two turns it provides just a little slack while breaking in.
After its broken in you can retention.

A Brooks takes a little sacrifice in the beginning, but by getting through the break in as I described above the rewards are awesome comfort for the life of the saddle and they do live a long time.