As stated, the quickest way to travel is through the Nether, but it is not on foot.

The fastest horse, [according to the wiki][1], 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`)


  [1]: https://minecraft.wiki/w/Horse#Health.2C_Jump_Height.2C_Speed