I hiked the AT in 3.5 months at age 51. I hiked the PCT and CDT earlier, so the 20 mile days were just my natural pace since that pace is pretty much required on the longer trails due to the shorter season.
Like Kaptain Kangaroo's experience, it was not a forced march at all. In fact, comparing notes with slower hikers I passed, it turns out I actually saw more than most. I had the energy to take more side trips. I was usually hiking at dawn and dusk, so I saw more wildlife. I had many great sunrise and sunset views.
I saw a lot of hikers spending up to 16 hours a day hanging around a shelter. I suspect they were among the ones taking up to six months. Huddling in the corner of a mouse- and snake-infested lean-to, downwind of a loaded privy, in the company of habituated bears and snoring hikers, is not my idea of a good hiking vacation.
Another good reason for a shorter hike is cost. An average AT hike costs about $1000 per month, and it doesn't matter much how many miles you hiked in that month. The opportunity cost has already been mentioned.
I met a few first-time hikers finishing in Maine on a four-month pace. They were strong, confident hikers having a great trip. It can definitely be done and it does not have to be difficult. Hiking 2 mph for 10 hours a day, the world isn't exactly flying by to the exclusion of all enjoyment!