"On your left" has always been trouble for me. It generally works OK in real bike events with semi-experienced riders, but it's death on MUTS. I avoid multi-use trails when i can, but my office backs right onto a nice one and it's perfect for lunch rides. At least half the time if I sweep along like an Arrogant Roadie Pri** and shout, "On your left," the old ladies or moms with strollers or little kids with training wheels either freeze or move to the left themselves.
Solution: Come up slowly behind them and say something like, "Can I pass on your left?" or "I'm going by on your left, OK?" Then wait to be acknowledged before you make your move.
If you want to go faster than that, you don't belong on the MULTI-USE trail.