It looked exactly like a gastrolith. You can find these pretty easily in Southern Utah. Dinosaurs are birds and they ate rocks which then crunched up the food in their gizzard. Once they were all shiny they would go down to the stream to gobble some more rocks up but puke the shiny ones up first. Sure looked like a gastrolith.
The run is to benefit the Cape Ann YMCA. Next year it will be the 75th running. Lots of famous people have run this race, Billy Rogers for one. I always liked Rogers since he said he enjoyed potato chips with mayonnaise. So do I.
I kept the rock.