Good workout plan to become great at rucking?

It's called TUR- Time Under Ruck. Just put your heater on and get out there and walk. The more rucking you do the better you get.

I do believe that being comfortable with a ruck is a big factor. Rucking shouldn't be something you only train for once but something that you should incorporate into your fitness regime. But working on other components such as leg strength will really help you out. If you don't want to get too crazy with a workout plan I would recommend progressively spending more and more time on a stair stepper. You should be doing it moderately difficult to where you are breathing hard. You will be surprised how much progress you will make with your rucking and running.

Start off slow, go for a 1-2 mile hike and practice just staying at a 15 min/mile pace. After that you can introduce weight, start low with 10-15 lbs then 20-30, then 35. Once you can sustain that pace for 3 miles then you can start pushing your milage up. You should only be rucking 1-2 times a week making sure to rest at least a day after. Warm up prior to each workout, hip stability drill is a must. Throw a podcast on or some music and just go at it. Be sure to pick varying terrain, keep your running/jogging with a ruck on to a minimum. Have a good pair of boots and arch support. Be sure to stretch after and before going to bed. Check out the SOF recruiting website and they have a good workout plan posted that helps ease you into rucking with gradual increases.