you are right Davebugg it is not average and it does require a good shape! With depends I meant age/physical constitution/weather and circumstances in the best way! 4month and 2200miles is okay, no?
I had days with more than +-2000m inclines and still 30miles a day, top was 40 miles a day. was a special period. An average of 20miles....couldn't do it again probably!
Neither the PCT or the CDT is 2200 miles long. The PCT is 2650 miles and the CDT is 3100 miles.
When talking about such things, I tend to look at the normative values. To do the PCT in 4 months or under is outside of the norm. Most backpackers that
are in good shape fall into the 'norm' which is around 5 months. So, yes, it is possible to do it in 3.5 to 4 months, but not for the vast majority of backpackers.
To do the PCT as a thru-hike, you have to start when the snow pack is still heavy in the Sierras when traveling north bound (NOBO), or in the North Cascades if traveling south bound (SOBO). Frequently, you are slogging through deep snow fields, crossing raging torrents of water, and trying to determine the actual route which is covered with snow and ice at the high elevations. If there is a late season snowstorm, or if the snowpack is heavier than normal, there are delays in progress that can last days or weeks.
Then there are the dozens of resupply stops that must be taken, which might be many, many miles off of the PCT, requiring either a long walk or hitch-hiking a ride into a town. Many times, these are combined as one or two rest days before traveling back to the trail. Sometimes it only takes a half a day to get into town and then back. A few times, the PCT crosses a resort area where you can resupply in a couple of hours.
The point being, that the time frame in which the couple thru-hiked the CDT, which is longer than the PCT, is not usual.