The CT is about 500 miles long and has about 90,000 feet of accumulated elevation gain, which averages out to 3,200 feet over each 18 miles. That's not especially much for mountain hiking. One thing that makes the CT a bit harder is that it lies mostly at altitudes over 10,000 feet, where the air is thinner. Hiking 18 miles/day is quite an effort in any case, but still doing it on a hike like the CT is something the average person can do if they set their mind to it and train a bit for it.