An extended hike is like a section hike, but longer. Extended hiking could be categorized along with section hiking, as there is no specific time or distance where section hiking ends and extended hiking begins. But in general, extended hikes are longer, at least a week and perhaps much more.
The distance that can be covered in a section hike or extended hike of a certain length of time varies greatly depending on the location. On relatively level terrain, you can go much further in much less time. On steep terrain, progress is much slower. Also, individuals vary in their abilities and their motivation. Personally, I am not interested in speed or distance. I find walking the Great Wall so interesting in so many ways that going slowly makes the experience more rewarding so that I can absorb and remember as much as possible.