Walking on treadmill?

32F, 182cm and 77kg. Decided to get fit, grow some muscle, avoid the dreaded skinnyfat body type I risk developing if I continue to live like a barnacle.

First order of business is get some more daily steps in. I set the speed to 4.5 - 4.8 and incline to about 9. The pace is comfortable enough for me to watch some YouTube and the incline feels like my thighs are working.

But whenever I go to check my heartrate on the handles, it stays around 75-79 which is... not a lot?

How important is heartrate? I read somewhere that it needs to in the 120 (70% of max heartrate) range to have any impact on fitness.

What do I do? I don't want to run because that hurts my joints.