ichheißeMatt

Hello all, I am learning about prepostions and having some trouble understanding when to choose which ones and how they work in context. Specifically, one of the practice sentences in my grammar book has got me stumped as to how they came up with the answer listed in the back of the book. The sentence, with the words filled in, reads as follows: Meistens fährt er mit dem Auto, manchmal geht er zu Fuß. Now, my expectation would have been for it to read "manchmal geht er mit Fuß" or "manchmal geht er mit dem Fuß." Why would it be that they have chosen zu rather than mit, despite the parallel construction? Additionally, I don't understand why they ommitted the article belonging to Fuß. Even choosing zu over mit, it seems as though the correct answer would then be zum, rather than simply zu. Any clarification would be greatly appreciated!!