The story is "Unpossible" by Daryl Gregory. It appeared in FSF October/November 2007. It may be read here. It features a middle-aged man who, during a midlife crisis following a family tragedy, tries to return to the nostalgia of his youth.
In his journey, he crashes into the Phantom Tollbooth with his recently modified car. He meets an older Dorothy Gale and Toto in her displaced Kansas house. After leaving her, in an attempt to re-enter the fantastic world he found as a child, he encounters an adult Max (Where the Wild Things Are) still wearing his footie wolf suit. Together, they learn a valuable lesson about childhood.
The main character might not be Milo (though Dorothy does refer to him as being possibly "the Tollbooth kid"), the presence of the Wonder Bike indicates that he may not be Milo. While I was unable to determine a children's story featuring a kid with a Wonder Bike, it's likely that the main character is just an amalgum generic storybook kid turned adult.
I emailed the author. He confirmed that the main character was not Milo, saying:
The character in "Unpossible" is the grownup hero of his own childhood
adventure story, which I made up. But thematically, he's a brother to
Milo and to Max in Where the Wild Things Are, and even Dorothy -- all
being kids who ride a magical vehicle to the other side.