I do a lot of treadmill training in preparation for the PPA. I would recommend you do the PPA treadmill simulation because it will provide some feedback on your progress rather than just walking at a 15% grade. I published the PPA treadmill simulation a few years back and I have expanded it for my own use. I log my time at each checkpoint and record my heart rate and note the calories/hour I am burning. (Note: not all treadmills provide this data. Matt Carpenter recommends a constant effort; slower on the steeper slopes and faster on the shallow. Here is an example:
Speed (mph) Grade
Here is a link to the chart I use to log my PPA simulation runs.https://1drv.ms/x/s!Ap1RwN8MNqtjiUun39DJ-OWbUysd
I live at 8,000', so it may seem easy to maintain these slower speeds at lower altitudes. If so increase the speed for each grade. You should be able to walk to Barr Camp (7.6 miles) at the time of this posting. As you increase the length of your long runs, you should be feeling strong when you reach Barr Camp. If you have difficulty completing the legs beyond Barr Camp, you need to slow your pace in the early legs.