I'm a trail runner and would like to record some of my runs. Would this harness work for that or would there be too much bouncing to make a good video?

You could also turn it around pointing backwards (off your back), which seems more stable for some reason.

You will get a little bouncing with this mount, but it would be one of the better mounts to use. The straps cinch down pretty well to hold the camera in place so it shouldn't bounce much more than your body does as you run. I would use the R3 setting to record, that way you could slow it down if you wanted to and it won't feel as bumpy. I use this mount dirt biking and the picture is a lot more stable than I thought it would be.