Those who sign up for the 15K Stone Tower Trail Run Challenge at Lynn Woods on Sunday certainly face a tall task.”If you’ve never run a half-marathon on the road, you might ask if this is the race for you,” said Liam Brady, a Lynn native and Danvers resident who came up with the idea for the event about four years ago.The day is an extension of the Lynn Woods summer series and also features a 5K race and a 1.5-mile walk to the tower itself.As for the 15K, “It’s brutal, it’s hard,” Brady said between laughs. “It has the ups and downs of Lynn Woods. It never repeats itself.” However, he noted, “You pass the stone tower three times.”The 48-foot-tall tower stands atop Burrill Hill, the highest point of the Woods (285 feet, according to a map on the race website). It will be open to visitors on Sunday. Yet it is far from the lone attraction of the 15K race.”The one thing that distinguishes the Stone Tower race is the course,” Brady said. “It ascends both the highest hills, Dungeon Rock, passes two reservoirs, the length of Undercliff, one of the lowest portions. People want that.”It sounds like quite a few of those people are trail runners.”People who like to work out don’t mind a challenge,” Brady said. “Trail runners are a different breed than road runners ? Road runners are more about speed. This is more (of) the whole experience.”Trail runners run in any weather. They rather enjoy being out in the woods. If they can’t be trail running, they’d still be hiking. They expect and anticipate that things will be a little more challenging. They know there will be rocks and roots. They’ll be heads-up about watching the trails.”The 15K seems to draw a good number of veteran runners. Of the 58 people who had pre-registered for the 15K by Monday night, 36 were between the ages of 40 and 60 years old: 23 men and 13 women. There were also six pre-registrants in their 60s, as well as a 70-year-old.Runners hail from not only the North Shore, but also destinations that Brady describes as “good far-flung places,” including Brooklyn, New York and Juneau, Alaska.The Stone Tower inspired the actual race. Four years ago, park ranger Dan Small issued a poster seeking ideas for raising funds to restore the tower. Brady pitched his concept to Lynn Woods summer race coordinators Joe Abelon and Bill Mullen. In its first three years, the Stone Tower race has raised approximately $12,000.”We try to keep it simple,” Brady said. “The T-shirts are the biggest expense. The design’s become a real fan favorite (it incorporates a Native American dreamcatcher as well as the tower).”Brady credited a number of people for helping keep Lynn Woods an enjoyable place, including the Friends of Lynn Woods, Abelon and Mullen.Sunday’s events begin with the 15K at 9 a.m., followed by the 5K and 1.5-mile walk, both at 9:15 a.m. All events begin at the Lynnfield Street entrance to Lynn Woods. The cost is $25. For more information, visit http://danvers5k.com/stonetower/