Rucking, walking with a rucksack or backpack that's loaded with 20-plus pounds, is one of the best forms for cardio or conditioning for lifters. Think it's too simple? Well, good. It ought to be.
Since lifting is mostly a game of high intensity work, lifters often suck at aerobic work. If you get exhausted climbing stairs or walking up a hill, your aerobic conditioning sucks. This area is your weak link. And improving this weak link will boost your performance (more weight on the bar).
By improving your aerobic work capacity, your anaerobic threshold will increase, meaning you can perform more quality strength and muscle-building work. You'll also experience improved recovery, both in between sets and between training sessions.
After you've loaded your rucksack, imagine you're going into nature, enjoying fresh air for a change, and seeing living things instead of being surrounded by buildings and "attacked" by stress (a typical scenario for most people). Since you have some weight on your back, you have to brace your core to counter it, which is great for posture and low back problems.
Compared to walking, rucking is said to be about three times as effective for caloric expenditure – more if you increase the weight and go faster. Your heart and lungs will also let you know how effective it is.
Rucking is comparable to low-intensity strongman exercises like the farmer's walk and yoke carries. The intensity is simply scaled down to target the aerobic system. If you're new at it, a 20 minute walk with a 20 pound backpack will be a lot tougher to perform than a short distance of heavy farmer's walks.
While HIIT certainly has its place for conditioning work, serious lifters are already living closer to that world, and hence, it's more familiar. Improve what you've probably neglected the most – your aerobic system.
It's simple, everyone can do it, and while I recommend going into nature to enjoy the silence and let the landscape dictate the intensity, you can do this anywhere.