First stab at an itinerary

Fly from London to Mexico City end of March
Mexico – 6 weeks (April)
Guatemala – 4 weeks (May) – including a Spanish course
Honduras – 4 weeks (June – including a diving course in the bay islands
Ecuador – 2 weeks (July)
Peru – 6 weeks (July / August) – including Inca Trail
Bolivia – 4 weeks (September)
Argentina – 4 weeks (October)
Australia – 1 week (October)
Indonesia – 5 weeks (November)
Philippines – 5 weeks (December)
Vietnam – 4 weeks (January)
Laos – 4 weeks (February)
Cambodia – 2 weeks (March)
Thailand – 1 week (March)
Fly back from Bangkok to London

Off to the travel agent this weekend to start discussing fares too.