how should i be running to burn the most amount of calories??

i use the treadmill about 3x a week and usually walk at 4mph for .5 mile, run 6mph for 1-2 miles, then 4mph for .5 mile. sometimes il change it up and run at 7 for a half mile or something... i end up burning around 300-400 calories.

can you do hiit on the treadmill? like walk at 4 mph for a while then run at 10 mph then walk and etc. or is it best to just run at a constant speed for as long as i can?? if hiit is the way to go, how long should the intervals be??

thanks for any help!