Los Angeles has has so many great places to run outdoors. So why are so many people paying to run indoors?

That piece of gym equipment that everyone loves to hate is now at the center of one of the hottest boutique fitness trends: treadmill running classes.

“I can run outside and do 20 miles … and then come in here and do an interval class and feel just as tired or more tired. This is the icing on the cake to make my training that much better.”

Rather than strapping on shoes and hitting the pavement to enjoy a free run in SoCal’s near-perfect weather, an increasing number of locals are plunking down as much as $30 a session to run inside on a piece of gym equipment they had probably ignored for years. In fact, says David Siik, developer of treadmill running classes for high-end fitness club Equinox and author of “The Ultimate Treadmill Workout,” a majority of people who walk into his classes for the first time immediately offer the disclaimer “I’m not a runner.”

But it’s these reluctant runners — and runners who want to run with a group but are afraid of getting left behind — who are fueling the boom in treadmill studios.