I am a huge believe in multi-tasking: laundry while doing homework, meal planning while watching tv, doing strength & cardio at the same time. Why? Because I am a grad student. Because I get bored easily. Because I believe in mixing business with pleasure... and by that I mean tackling my never ending to-do list while doing something enjoyable.
I know it is good to change up my workout and I like walking but I get bored easily. I don't always have time to plan my workout around a tv show I may want to watch. Why can't I combine walking in the treadmill with upper body strength exercises? All I need is a pair of 5lb weights and a theraband.
I would personally walk at 1% grade at 3.5mph but you could easily make it harder or easier by changing the grade or the speed.