6 January 2017 Last updated at 07:14 GMT
Louis Nethercott, 27, from Wiltshire, was medically discharged from 42 Commando in Bickleigh, Devon, last year after struggling to cope with seeing his colleagues killed in Afghanistan.
Alongside another former Marine, Anthony Lambert, the pair are back in the UK after completing a gruelling trek across Borneo.
Louis, who grew up in Bristol, has four more islands to go - Papua New Guinea, Madagascar, Greenland and Baffin.
A former Royal Marine who's recovering from post-traumatic stress disorder has completed the first stage of a challenge to cross the world's five largest islands unaided.