Richard Adams wrote it based on his military time. Certain features of the book are clearly informed by his experience, like how they're constantly talking about how fatigued or rested they are, based on the speed they've been traveling or working and how long they've been at it.
He said he based particular characters on particular people he knew. The seagull was a big explosives guy, Bigwig was a tough-as-hell officer that he really liked working with, and so on.