Merson set a new personal best of 1:06.03 on the new-look Plymouth half course.
That was 20 seconds quicker than he had run at Bath in March.
Cornwall AC veteran Colin Snook took second place in 1:12.58, with Tavistock Run Project’s Jordan Andrews third in 1:13.10.
The women’s race was tight, with Tiverton’s Serane Stone edging out Alexa King and Cornwall’s Emma Stepto. The top three women, who were all over 40, all finished next to each other. Stone clocked 1:22.56, King 1:23.13 and Stepto 1:23.14.
Erme Valley Harriers’ Alison Thorn and Alison Engledew and Tavistock AC’s Jodi Fisk, Sam Lake and Tracey Oxborough all claimed top three age category placings.
More than 3,000 runners completed the 13.1 mile race that started and finished on the Hoe and took athletes out to Elburton and into Saltram Park.
Full results can be found HERE.