I won't belabor this too hard. There are a lot of resources. These are just my thoughts on how it worked for me to go from walking an hour or so a couple times a week to chasing down and smashing the 12Miles in 3 hours ruck standard with a 55 pound(in my case 75 pound) pack...while still powerlifting.
This took a year and a half.
I started by walking fast...I'd measure cardio in halves of an hour. The minimum amount is therefore 30 min. An hour and 23 minutes is 1 hour.
I added a weight vest once I could keep a comfortable 3MPH pace. The starting weight was 20 pounds.
I would add between 4 and 8 pounds each week or so but would also back cycle. work up to a 40 pound vest, then drop back to 30. I did this entirely on feel.
I would nearly always Ruck after squatting, sometimes the day befgore but always after. (my squat went up during this time)
Once I was pushing a pace around 3.5 MPH with a 40 pound vest, I switched to an ALICE pack.
I stayed at 40 pounds for a solid 2-3 weeks. then slowly started adding.
At this point, (1 year ago) I was gettign 8 plus hours of rucking in per week. Never less than 1.5 hours at a shot. Sometimes 3.
I did not make the route intentionally hard. I would walk flats, hills, whatever but let my recovery dictate the terrains.
At this point i started working with a new coach and we tried a number of things, including intervals on the bike, running etc. We also tried running with the pack. Do Not Do This.
By March I was hitting 9-12 mile rucks easily. Able to hold a 4plus per hour pace off an on while carrying between 50-80 pounds all the time. I felt unstoppable. My resting HR was back in teh low 50's, BP dropped. Appetite was through the roof. My lifting was unaffected until....I did on long hard test loop with a very heavy pack cxhasing that 12miles in 3 hours target. To stay on pace I had to run. This caused the shoulder strap to crush some nerves in my brachial plexus.
I got this...
I lost use of my tricep, delt, suprpinatus and a few fingers for a time. I went form benching 315x8 on New Years to gettign stapled with 135. It's been a long year rehabbing this.
Keep load moderate , use the hip belt.
It's a very fast awkward shuffle, do not run unless chased.
Footwear...figure it out.
look for calf or shin pump. usually means bad load distribution on pack.
Increase volume by time, not distance.
Do not jump weight up more than 5 or so pounds a week.
Once of the reasons my shoulder injury was bad is that the years of benching had built a lot of mass to impinge the nerve within. If you are heavily muscled, be very fussy about straps. always wear a hip belt.