Why not try running? If you think you aren't getting a good workout walking on the treadmill, but your walking form is bad at higher speeds, add some running breaks to your workout. Running will give you extra bursts of higher heart rate and a change in your form.
To add running intervals, first warm up at an easy pace for 3-5 minutes. Then increase your walking pace to the pace at which you are going fast but still are able to maintain good walking form.
Now start a jog and increase the speed to match your jogging pace. Continue to jog for 1-3 minutes before going back to your fast walking treadmill speed and pace for 3-5 minutes before doing another jogging interval. Repeat until the end of your workout, and finish with 3-5 minutes at an easy walking pace to cool down.