Griffiths's debut marathon was at the London Marathon on 23 April 2017. Representing his club, Swansea Harriers, he started the race alongside other club runners, behind the elite athletes. He finished the race in 2:14:49 in thirteenth place overall, the first of the British runners. His performance gained him qualification to represent Great Britain at the World Championships in London in 2017. The marathon was held on 6 August, and Griffiths completed the race in a time of 2:20.06 to finish in 39th place.