You are puzzled about the absence of "on". "Hike" is a verb and "trail" is the object of the verb. You could insert "on", in which case "on the trail" becomes a prepositional phrase.

The preposition "on" can be used but it's usually used to mean that we will adhere to the route of the trail. Often, I hike off the trail. Off-trail hiking is permitted in some areas, but it may be risky or in violation of an agreement.