But...I know my weight loss was all water...not fat or muscle.
Right, it was water, some from sweating and some from burning glycogen which is stored with water. And actually 8 pounds isn’t all that extreme, oddly enough.
For reference here’s a runner’s forum where they’re talking about weight loss during marathons and other long runs. 8 pounds is about the norm. One guy mentions 15 pounds was considered the safe upper limit during his high school football practices.
https://www.letsrun.com/forum/flat_read.php?thread=3319287