I see them several times a year. Sometimes more than one in a week. Still doesn't make me an expert, I don't think. But, what my friends and I do is the same as Fort Fireman says. We don't back down, definitely don't run, make ourselves big, and all of us roar, wave our arms, get quite ferocious. You may be surprised at how motivated you get when faced with one.
Mtn. Lion stalk from behind, which kind of stinks. So, whether riding or walking, we frequently check our back trail. And, unfortunately, I have been horseback and the horse and I have been both surprised by lion.