Many runners cringe at the thought of doing a treadmill workout alone.

But group speed sessions on treadmills—led by experienced coaches in upscale studios—are quickly gaining followers across the country.

The Mile High Run Club in New York City opened just more than a year ago and already has done nearly 3,000 classes. There has been such demand that some sessions have waiting lists, says owner Debora Warner.

“It’s been a great year,” she says. “A very strong start.”

Group interval training on a treadmill is so appealing because runners benefit from the class atmosphere without the worry of being too fast or slow for the pack, she says.

Combine that with enthusiastic coaches, high-energy music and glowing lights, and runners are hooked.

“If someone is going to embrace the treadmill as a training tool, then why not make it the best it can be?” Warner says.

RELATED: How to Make the Most of Treadmill Training

The studio is doing so well Warner plans to open a second location with 35 treadmills in the coming months. Mile High will take over the 3,300 square-foot space in Manhattan where TheRUN, a competing treadmill studio, operated until recently.

The success of Mile High has sparked new businesses outside of New York.

MyStryde is scheduled to open on Hanover Street in Boston early next year, says founder Rebecca Skudder, 27.

Skudder says the Northeast’s brutal winters gave her the idea several years ago, but she wasn’t prepared to start her own business then.

“I was training for a half marathon in the winter,” she says. “I especially remember getting on the treadmill, and I had a seven-mile tempo. I thought, ‘I can’t do this.’”

Since then, she wondered whether treadmill gyms could catch on like indoor cycling studios. The success of Mile High has convinced her they can.

“Once we saw Mile High open up, I said, ‘Yes, it’s right. If we don’t open now, someone else will,’” she says. “It proved that the idea was A. doable and B. popular—people were into it.”

MyStryde will have a dozen treadmills, including one for the instructor, and offer speed and incline interval classes, ranging from speed walking to marathon training. Workouts will take 45 minutes to an hour and 15 minutes.

For example, one class will include a 5-minute warmup followed by a 30-second hard effort at your 10K race pace and 30 seconds of recovery. The next hard interval includes a 1-percent incline and a .3 increase in speed. The hard intervals increase to 60 seconds as the speed and incline are also pumped up.

Set to upbeat music and taught by running coaches, the classes will be both fun and challenging, Skudder says.

“It’s something I always wanted to do, and it’s something that means a lot to me,” she says. “Hopefully it’s the right time, and Boston is the right city.”

Indoor running classes are also picking up speed on the West Coast.

Samantha Hubshman, 30, used to dread the treadmill until she started taking running classes at Equinox in West Hollywood. There she met David Siik, who was developing Precision Running, a treadmill program that Equinox launched last year in dozens of clubs across the country.

“Physically you get a workout, but mentally it keeps you totally engaged,’ she says. “I say to myself, ‘Sam, you can do anything for 60 seconds.’ Having it interval-based keeps you engaged in a whole new way.”

Now living in New York City, Hubshman goes to Precision Running classes on the Upper East Side several times a week.

“It transformed my life, and the way I see working out. It’s something I love to do, not something I have to do,” she says.

Siik’s program takes a low key, meditative-like approach to treadmill running.

“It’s not just about the experience, but also the actual effectiveness of it,” says Siik, 35, the national manager of the program for Equinox. “We don’t use music in the class, which surprises people. If you are distracted by music, you won’t be as connected to the flow of the class.”

The sessions follow certain speed and incline patterns to create a balanced workout, he explains.

In one advanced workout, called Fire and Ice, runners alternate between 60- and 30-second hard efforts, with variations in incline and speed, and take 60 seconds of recovery.

The interval changes are complex, but “they are designed to be very mathematical and extremely meaningful,” he says.

Siik, author of “The Ultimate Treadmill Workout,” due to be released next month, says the rise of treadmill classes and dedicated studios is a sign that people want simplicity.

“People get confused and lost in their fitness—they don’t know what is right, and what is wrong,” he says. “Running is having a resurgence. People are hungry for it. They are hungry to get back to what works. Indoor running creates a safe place.”

RELATED: The Tricky Art of Treadmill Pacing