Well, the way the military does it is mostly calisthenics and constantly pushing your limits.
An unscientific and possibly unhealthy form of what is basically HIIT and Zone Training. If I were you I’d do the same but be more scientific about it. After all if you fall out in the gym I doubt someone yells Corpsman and you get help in 1 minute or less.
You can can actually be better with less. Focus less on time and distance. Focus entirely on efficiency. Do the next-level-best as hard as you can for 100% of the time you are trying. You already have a solid base in body weight and calisthenics.
I only learned what this this really meant when I went to MCRD Parris Island and had my kidneys almost shut down. Once I was home I started playing with this idea and pushed it too far and broke both legs endurance running.
And now I’m here learning to love cycling because I focused too much on extending that time. Finding time away from family to get an extra 30 minutes and push a little further. When in reality I could improve in the first 30 minutes if I put out a true 100+% effort. More important, OP what are your goals? What is your purpose? That’s how you figure out how to improve. Constantly ask “is this directly pushing me forward”. It’s easy to get lost in training regimens.