As you have probably already realized, there is not one uniform rhyme pattern throughout "The Midnight Ride of Paul Revere." Longfellow's primary concern in writing the piece was creating the legend and telling the heroic story of the events of that night. Pure literary format was a secondary concern, with devices of word choice and length of stanzas used to support the action rather than to fit a given writing structure.
For the most part, the rhyming scheme is based on pairs of rhymed lines. These pairs may succeed each other (aabbcc), may appear at different points within a stanza, and may include more than two appearances of a particular rhyming sound.