So it seems to be the trend is ..
-- You can do a lot more than you think in a day for a day or two
-- You can train up to more in a day over a couple months
-- Marines are tough
-- You can train prior and it can help
-- Have fun hike your own hike
-- There is a certain weight .. where speed seems to get impacted. I would generally argue same is true for terrain etc
-- Stacking huge days .. will only work for a couple days
In the end .. know your capabilities. If you want more then train for them as close to possible, or plan to work up to them if taking the "long" view.
And .. enjoy it . Try to avoid making it work as much as possible . Training can help increase distance and speed covered if that is the goal.