I can go out my front door, turn right and ride up 4200 feet in the first 16 miles.

My advice is find a power level you think you can handle for the length of time you plan to cover that 3000 feet and find a gear that lets you push that power on flat ground and train in that zone. My advice would be to try and stay at about 80% of your ftp for the climb to let you get use to both climbing and the altitude.