Jim McCarvill had just finished an adrenaline-fused ride on his mountain bike down a twisting bobsled-like trail at the nationally acclaimed Cuyuna Country State Recreation Area.

“I absolutely love it,’’ said McCarvill, 64, of Richfield, an avid mountain biker who traversed the rugged trails here nearly a dozen times last year.

He’s among those not surprised that thousands of two-wheeled enthusiasts have been flocking to the former iron-mining area of Crosby, Ironton and Cuyuna, two hours north of the Twin Cities, since the 25-mile trail system opened in 2011. Visits to the 5,000-acre re­creation area, which also boasts boating, fishing, camping, scuba diving and hiking (bike trails are open to hikers), have increased from 118,000 in 2010 — before the trails opened — to 164,000 last year, up nearly 40 percent.

“People all over the country are coming here and loving it,’’ McCarvill said before pedaling off.

Erika Rivers, new head of the Department of Natural Resources parks and trails division, also isn’t surprised.

“It’s on the map nationally as a destination,’’ she said. “We’re seeing people from all over. It’s one of those sports that’s really ‘next generation.’ The 18- to 30-year-old demographic has embraced it.’’

As have older bikers.

Cuyuna Country has become so popular that work began this week on a $1.2 million project to handle the growing number of visitors. Another 96 parking spots will be added, along with changing facilities, a picnic shelter, bike-wash station and restroom.

“It will redefine the park,’’ said Nick Statz, trail manager.

Meanwhile, three yurts — round tents set on insulated wood platforms and furnished with bunk beds, tables, chairs and a wood stove — also are being built near one of the trails, adding another attraction to Cuyuna. Available year-round, they sleep up to six, and bikers will be able to hit the trail right outside the door.

Bikers are embraced

Not everyone envisioned an old-iron mining area of northern Minnesota becoming a mountain bike mecca.

“The community was skeptical,’’ said Statz, one of the founders of the Cuyuna Lakes Mountain Bike Crew, the local mountain bike club instrumental in developing the trail system on abandoned mining land and responsible for much of the maintenance. “They had tried different ways to pump up the economy after the mines closed, with limited success.’’

Now the towns believe. Signs welcome mountain bikers, and vehicles loaded with bikes are common.

The recreation area’s 25-site campground often is filled on weekends.

“It was never a problem getting a campsite’’ before mountain bi­king arrived, said Jenny Lust, a park employee. “Every weekend was full last year.’’

Said Statz: “It’s been a huge economic boost for the community. It’s not a fad. We’ve seen steady growth.’’

And the bikers don’t stop coming when the snow flies. Statz packs the trails in the winter using a two-wheel-drive motorcycle. Die-hards on fat-tired bikes come cruising along, ignoring the cold.

“Some people think that’s crazy,’’ said Aaron Hautala, 38, of Cuyuna, president of the 75-member local mountain bike club. “But winter ri­ding has grown every year.

“It was 16-below one time when I was out there,’’ he said. “The key is dressing right.’’

Hautala is a relative newcomer to mountain biking, taking up the sport in 2011 after moving to Cuyuna.

“I got on these trails and said, ‘Holy cow.’ It gave me the same experience as downhill skiing in the mountains. I fell in love with it. It’s become a lifestyle; I just want to ride. I used to golf with some guys; now we all mountain bike.’’ A novice explores

One of the allures of Cuyuna is that its 25 miles of single-track trails cater to a diversity of skill levels. They are rated easiest, more difficult, most difficult and extremely di­fficult. (There’s also the 6-mile paved Cuyuna Lakes State Trail.)

While hard-core mountain bikers love the challenging trails, beginners and novices can find good riding, too. “You don’t need to be an expert,’’ Hautala said. “I’ve come a long way, but I’m still very much learning.’’

I’ve ridden some mountain bike trails over the years, with a 25-year-old bike that has no suspension. But I consider myself a novice, and, at 61, I wanted to explore the backcountry without wrecking myself. So I bo­rrowed a better bike from a friend, and we headed to Cuyuna last week. He’s an experienced mountain biker who has biked Cuyuna several times.

“We’ll start on the easy trails,’’ he said.

But it wasn’t long and I was tackling more advanced trails, and even the ‘most difficult’ black diamond trails like “Ferrous Wheel’’ — a nod to the area’s mining past — and “Bobsled” — aptly named for a series of banked curves on an exhilarating downhill run.

Along the way, we encountered a group of 10 women, with two female leaders, on a two-day adventure outing. Only two of the 10 had mountain biked before, but all were smiling.

“We’re so happy to get new people out here,’’ said Katie Seipp-Deblock of Ironton, one of the group’s leaders.

After exploring the area for a couple of hours, we later biked with Hautala and McCarvill. The well-marked twisting trails offered a variety of experiences, from harrowing to serene. The scenery is spectacular, and if you didn’t know the history of this land, you might never guess this is a reclaimed mining area. Some of the trails follow the steep edges of the former ore pits, and the overlooks provided views that rival those of the Boundary Waters Canoe Area Wilderness.

The uphills will take your breath away.

So will the downhills.