I find that on your left passing on your left or even on your left works better. I my self say coming up on your left. To your left sounds like something you may say to a riding or jogging partner when you see something of interest. Remember mups tend to be very wild life heavy with nice scenery many people on long ride or walks are there to enjoy said wild life and or scenery and will tend to assume you are as well.
So avoid conversational phrases like to your left or to the left.
For the record i hold a average speed of 17mph on my commute by bike to where i work on the towpath wich is a nonpaved mup