I mostly walk the flat trails but I try to run the uphills. There’s a couple that are pretty steep that I’m typically moving pretty slow at the top, but still going! The few looong ones I walk, my heart rate is pretty much the same if I walk the long ones or run the steep ones.When I first started, I had to stop and rest once or twice while walking if I was on a serious hill. I just kept challenging myself to do a little more each time. I've finally reached the point where my cardio is maybe 60% jog and 40% walk. I don't think I'll ever get to 100% jogging, at least where I live. There are way too many hills where I live, and I'm not tough enough any more to run up those big ones.