I'm also a volunteer firefighter and the "pack test" level of Work Capacity Test for wildland firefighters is 3 mi on flat ground carrying a 45 lb pack in 45 minutes.

It is pretty important to be in shape as you are often carrying a lot of gear and don't want to bonk and cause an issue that would jeopardize yourself, your teammates, or the mission.

Edit: to answer your second question, my wife and I hike recreationally just about every weekend and the team often hikes during trainings and does a weekly casual hike as well.