In the northwestern part of Koh Samui, one can put together a quite nice run incorporating two piers, a runnable beachfront road, and a small but scenic in Nathon, the original capital of Koh Samui. Especially nice at sunset!
The route follows Chonwithi Road, which is probably the best road along the water on Koh Samui. There’s a decent sidewalk and nice water/beach views. The road itself is 1.25 km long, and the two piers add 1.5 km. Just south of the end of Chonwithi Rd. is Nathon Beach, which is ~0.5 km long and lovely for running (best at low tide).