"All the land" must include Mount Everest, right? 29,000 and a few feet above current sea level, plus at least another hundred feet to ensure it's good and submerged. It's simple to divide that by 40 and determine that it had to rain almost 730 feet a day - over 30 feet an hour, over 6 inches a minute. 6 inches an HOUR is considered a horrendous downpour, usually associated with Category 3 and above hurricanes. The storm clouds required to support a drenchpour of that magnitude would extend thousands of miles out into space, and contain about 2 1/2 times the total amount of water that exists on the entire planet.
You might be taking all of the land in the world a bit too literally. For example there are many descriptions of Alexander conquering the whole world, when in fact he conquered a tiny part of it. But to people living in that time with limited transportation capabilities, he did indeed conquer the whole world. The description of Noah's flood would have also been written by people with very limited transportation capabilities. If the majority of the middle east was flooded, as far as the Noah and his family were concerned, that would be the whole world. It certainly would have been the whole of their world: their homes, their trading partners, etc... The only mountain that we know for sure that would have been in the flood region would be Ararat as that is specifically mentioned in the text and while people generally refer to Mount Ararat, the actual text is plural Mountains of Ararat referring to the whole mountain range. The highest of the mountains of Ararat, Mount Ararat itself is only about 17,000 feet but by the time they found it, it (and at least some of the other mountains in the range) were above water. Who knows how long it was above water, perhaps it never did flood.
But while interesting from an intellectual point of view (and fun to discuss) the exact details of how the flood occurred, the exact area covered etc etc aren't really important. The important part is what it teaches us about the nature of God and God's expectations of us.