So far they have cycled about 10,000km and hope to break the previous world record of 38,143km.
"Why, we hear you cry! Well, we still can't quite explain why we are doing this - but we will have 18 months to work that out," they wrote.
Kat Turner, a 31-year-old Masters graduate in International Law and Criminal Justice, said she was not an intrepid adventurer - "in fact I can't even ride a bike" - but decided to do the journey to experience different cultures and new perspectives.
Steve Turner, a primary teacher, said he was a "have-a-go dreamer" and was probably to blame for the trip. "This really is a plan of epic proportions beyond my experience.
"Besides, this time there will be two of us. I can't believe Kat agreed to it. However, the road beckons us, you only live once and this is one hell of a way to live it."
The couple hope to raise money for charity Practical Action, which aims to build the technical skills of poor people in developing countries.