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There's a trail I like to run with varied terrain mostly consisting of hills, would it be okay to do 600m resets there for a base building HIC?

I don't see why this would be an issue. Just stay within the boundaries of the HIC. Go all-out for 600m and rest completely in between efforts. Hills en route aren't going to make a difference. If you're thinking of changing it up and making it a fartlek so you're jogging in between all-out efforts, then that's a different story. It's no longer a 600m reset, it's fartlek.

I prefer this sort of running to track style intervals. For military/ leo, dare I say this would be more "sport specific".

I personally prefer running over varied terrain for most of my conditioning work.