I had a problem with a 5k on a bike path last year. The race director had actually asked the city whether it was OK to put paint on the bike path. She received an emphatic “no.” She was told that marks adjacent to the path were OK, but since this was gravel and grass I didn’t think it would hold paint all that well.
I wound up using wooden dowels, one inch diameter. Each was painted yellow on the end, and drilled to accommodate a PK nail. The ground was very hard, with lots of stones in it. I had to use an iron bar to beat each hole deep enough to allow the dowel to be flush with the ground. I wanted them flush so the lawn mowers would not chop them off. It took lots longer than just painting would have done.
As I worked I took photos of the splits to help them find the markers. I was not there on race day, but I did not hear of a problem.
Paint on the path would have been a lot better. On bike paths I generally use a four inch diameter dot, with no adjacent number. This has not yet caused me trouble. On streets I usually use a painted number.
The finish line was described “Finish – 5 km – On the Jim Simmons Trail, 90 feet west of the sign marking the lower end of the trail. Marked with a wooden stake containing a PK nail, installed flush to the ground on the north side of the path.”
The photos are shown below.