^^ My commute is less than yours. Slow and steady will get you more benefit IMO. As soon as you start speeding up you'll start burning glycogen rather than fat.
The fat burning zone and max fat loss don't necessarily correspond. Higher intensity training can have better oxygen burning while you sit. Going as fast as is comfortable is fine.
Try going balls to the walls fasted is asking for unpleasantness, but you don't have to toodle either :)
^^ My commute is less than yours. Slow and steady will get you more benefit IMO. As soon as you start speeding up you'll start burning glycogen rather than fat.