300 ft Ethernet run parallel to 120v circuit
Since the electrical wire is in metal conduit, I would have no fear of interference. Just do not attach the ethernet to the metal conduit, as you are not allowed to use electrical conduit as a hanger for anything.

Another factor:  Most plastics do not like UV light, which the sun emits in great quantity.  This could be a serious problem for the sheath on your ethernet cable, depending on placement. Cable rated for outdoors or for UV exposure will last longer, but will still fail in direct sunlight.  

(I think, due to your concerns about interference, you wouldn't think to put the ethernet inside conduit carrying 120/240/480V power cables.  But just for reference, that is not allowed unless you use transformers to step down the AC voltage significantly, which would not work unless your loads were quite small.)