The good news is that the more training you do the fitter you will be for heading to Denali.
With 12 weeks you should do the first 8 weeks of the base period with their Max Strength workouts. Then do 6 weeks where you are doing one ME and one Max strength/week. Then go back to 6 weeks of Max strength. This will really set you up for the final ME phase.
As long as you keep the aerobic base training volume up doing this much ME will not result in a drop in the aerobic capacity. Its really when the z1-2 hours begin to drop in relation to the ME that you may run into issues.