5.7% grade works out to 300 feet of elevation gain per mile, and they built that into this road for 13.3 miles:
Unfortunately I woke up with a stomach flu thing on Friday, and by Saturday morning was feeling ok but not so great. At the beginning I felt weak and briefly thought of aborting the ride, but mentally decided to buck up and did this one at the bottom of zone2 for a 60% IF.
I’ve done similar HC climbs at 85% IF - disappointing that I didn’t feel better and definitely not a Strava ego stroker.
This climb turned into a basic, fully aerobic, nose breathing, moderate torque 66rpm grind to the top. So with your detraining, no issues coming to visit Quasimoto 
for some similar climbs plus I had chickenpox as a kid and recently the shingles vaccination.
Mount Shasta is the 5th highest peak in California at 14,179 feet / 4,321 meters, but the road tops out around 7800 feet / 2377 meters. It has the most volume of all the stratovolcanoes in the Cascade Range which runs from Northern California thru Oregon, Washington, and Southern British Columbia. Here is a infographic from Wikipedia:
After I posted pics at the top, descended and near the bottom stopped for a picture of nearby Black Butte:
This area has inspired and drawn people from time immortal. Remember the Harmonic Convergence that happened right after I moved to California? I sure do - Mount Shasta was one of the power centers.
After the ride we went to the (proposed 51st state) State of Jefferson Beerfest (now the ‘River and Rail Brewfest’) and my wife’s Ultimate High School Reunion:
Gorgeous. Unspeakable beauty. Less than 10,000 people between the 3 ‘major cities’ that flank the western slopes of Mt Shasta. No wonder that area wants to secede from ‘the real northern’ California and southern Oregon.
Next morning we took a 3 mile walk along the Union Pacific railroad tracks, like in that movie Stand By Me:
along with other hikers to see Mossbrae Falls:
Saw some interested cloud formations:
and reluctantly pointed the car south for the 3.5 hour drive home.