(This is the second of a series of articles on local runners who will be tackling the Boston Marathon this year)In retrospect, Mike Derby says he knows how his right knee began to hurt him.Back in September, when the Lynner and veteran of 20 consecutive Boston Marathons was training for No. 21 at a “boot camp” in Wakefield, he was working out with what he described as “a very large rope,” comparable to one on a ship.”I had to take it and lunge to my left and right,” Derby said, “with motion back and forth. The next day, I went on a run and I felt pain in my knee.”He said he thought it was tendonitis and that he could make his way through it. Yet by the first several weeks of November, when his Marathon training was slated to start, “it was not coming around,” he said. And when he went for an MRI at North Shore Sports Medicine in Peabody, the diagnosis was a torn meniscus.”It was kind of strange,” he said. “It would flare up on some runs, but it was always there – a nagging injury ? Some days were better than others. I was limping a couple of days. It was something more than just tendonitis.”I never had any injury before,” he added ruefully.In December, Derby, 57, underwent surgery in Beverly, and it was discovered that in addition to his torn meniscus, he also had arthritis that had to be treated.”It’s been quite a season,” Derby said.As a result, he has had to revise his plan for the Marathon to a pace of running two miles and walking one. A 3:1 miles run/walked pace would be ideal, he said. His goal is to finish within six hours.When it was time to talk about a recovery period from his knee troubles, Derby was told that after four to six weeks, he could start jogging and “get back to somewhat of a routine,” he said.”I started out and felt pretty good,” he said. “After six weeks, I didn’t pace myself. That set me back a couple weeks. Four or five weeks ago, I took a cortisone shot so I could do at least eight miles, 10 miles, 12 miles.”If it sounds like Derby has pushed himself, there is a reason. Running has served as a motivation for him for over two decades. He started long-distance running in 1988, as a means of coping after his sister Linda died of liver cancer. For the past 17 years, he has run the Marathon for the American Liver Foundation. His efforts included a fundraiser at the Lazy Dog in Wyoma Square last month, which he said was “very successful.”It has also been a tough training season for Derby’s running partner, Terry O’Brien of Wakefield. (They both work for the same company, ENE Systems of Canton.) She has had to deal with a foot injury and, like Derby, is planning on a 2:1 miles run/walked ratio and 6-hour finish. She is running this year’s race in the wake of her mother’s diagnosis with stage 4 liver cancer.”It’s another reason to run,” Derby said. “Almost everyone you talk to knows somebody (with liver cancer).”Between now and April 18, Derby plans to train on the elliptical at Mike’s Gym in Wakefield, 45 minutes a day. On weekends, he is looking at “some type of endurance,” possibly on a stationary bicycle, for two to three-hour stretches.If he finishes this year’s race in six hours, it will meet the qualifying time for the 2012 Marathon. He would like to earn Hall of Fame status by running in 25 straight.”Miss one, and it’s all over,” he said. However, he added, “To get across the finish line, whatever it takes to do – that’s all I want.”No matter what time he finishes in, he can count on seeing his wife, Lisa, and sons Mike and Matt at mile number 26.2 in Copley Square.”She always encouraged me,” he said of his wife. “She’s my beautiful bride. She put up with all my stuff – fundraising, and more doctor’s appointments than I’ve had in my entire life.”