What do you do to stay in shape for ruck marching?

This may be an obvious answer, but the best way I have found to stay in shape for ruck marching.... is just ruck marching.

The elements needed for ruck marching, such as strong legs to avoid bone fractures and shin splints, tough feet to avoid blisters, and of course strong shoulders and back to carry the ruck, cannot easily be replicated with anything but ruck marching.

I suppose one could do crossfit and high rep squats to support staying in condition for rucks, but that would not necessarily cover the toughness you need in your feet and bones.

Proper equipment is also a key element of good ruck marching, and this includes both socks and boots, and anything else you learn about yourself that you need (mole skin, sock liners, insoles, etc.). Until that equipment is put to the test, you may not be setting yourself up for success. There is no amount of non-ruck preparation that will help you avoid the problem this fellow has in the picture above...

To stay ready for a ruck, I ruck.

I do at least 1x 5K per week. And I invest in mole skin.

I had a team leader that swore by using a Stair stepper or an endless step machine. He was from RGR Bat and ended up going to RRD before it became RRC. I used it and my ruck vastly improved. Beside that, as LTC Yinon Weiss said rucking itself is the best way to improve your rucking.