I hike uphill mostly, and it’s pretty easy to stay in the zone with the right grade. You may have to experiment to find the sweet spot. Like yesterday I hiked up a ski slope, and it can get kind of steep so I had to either slow down on occasion or zig zag to keep my HR down. But a slightly less steep grade and it’s pretty easy for me to keep my HR in the right zone.
On flats I alternate walking and running. Also last year I spent a lot of time learning to jog at a lower heart rate. At first I could only do that going downhill but eventually I could jog back and stay in my zone (this was on a gentle grade, like 4%).