As stated, the quickest way to travel is through the Nether, but it is not on foot.
The fastest horse, according to the wikiaccording to the wiki, moves at about 14.5 blocks/second. In the Nether, where players only have to travel an 8th of the distance as the overworld, a player traveling straight to 0,0
from an overworld point 5 million blocks away could do so in no less than 12 hours.
As mentioned in the comments, this is a long time, and travel would obviously not be this quick, but it's doable over several days, especially if the user knew where they were headed (IE, 0,0
)