So, rucking is effectively a steady state exercise. Which is fine, in a lot of ways that’s actually ideal in the sense it is incredibly low impact, and manageable for decades. If you did only that for your entire life, you would manage a great physiology. I am a huge proponent of the average person sticking to something they can commit to regularly for the course of their life. Far too many people get gung ho for 6 months and burn out. So if rucking is what you enjoy, fuck yeah!
The main difference is, HIIT is made to intermittently bring you above your lactate threshold. As to say, just to the point of discomfort. That acidic burn in your muscles when you’ve worked at near maximum exertion is the “feeling” you’re looking to achieve. The constant flux of achieving that acidic burn, and then resting, all to achieve that burn again, has very strong physiological influence in meaningful aerobic capacities.
There is some possibility incorporating 1 HIIT session into your routine would actually increase your rucking distance. Especially when your distance is around 3 miles or so.