I did a pretty extensive search for Belize, Cozumel, Progreso and Costa Maya (we are cruising Western next weekend) and didn't come up with much.
I found several mountain bike tours, but the more serious ones (my dh is an avid biker and racer) were 2 or 3 day tours. Carnival offered "Biking in Belize" as a shore excursion, but I'm afraid my dh would go CRAZY riding at the tourist pace.
Let me know if you come up with something - I'd be curious.
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