If you're used to seeing を as marking a direct object, and having it mark an area passed through seems strange, consider this: Even English does this occasionally to what grammatically ought to be direct objects, as in "travel the world" and "sail the seven seas". While you wouldn't normally say "I walked the park" (#889) in English, it's the same concept.
を is a particle which is essentially acusative case. It also serves as connective of opposing thoughts,as shown in #4962.
The three uses of を [wo] are 1. objective / target (目的) 2. area passed through (通過点) 3. departure point (出発点)
を is used to mark a noun or noun phrase. N = noun.