Short answer: yes, you can
Longer answer: The reason we have the prerequisite requirement for longer or more difficult rides is because too many folks were attempting the harder ones as a first certificate ride and ended up biting off more than they could chew. If you first complete a Bun Burner 1500 (even if you haven't yet submitted it), you will have an idea of what the longer ride will take. So proceed with your plans.
Ira Agins
Iron Butt Association