Have you ever tried walking uphill for 30 minutes? It's tough.
Here is what I recommend.
Warm up for about 3 minutes on the treadmill at about 3.0 and increase the incline every 30 seconds or so. By about minute 4 or 5 you should be at an incline of 12. The highest incline you will reach and maintain.
For the next 30 minutes remain at the incline of 12 and play with your speed between 3-3.6mph.
image from www.weheartit.com