Until then, he had never really considered his smoking habit.
By the time he was in his early 60s, Derek had been smoking about 20 cigarettes a day for 40 years.
He was a keen footballer and played up to three times a week for his local club in Wakefield, West Yorkshire.
"I never saw smoking as being a problem," says Derek, a community fundraising manager for Heart Research UK.
"I trained and played football regularly until I was 43 and I mistakenly thought that doing sport counteracted the smoking."
When he stopped playing football, his 20 cigarettes a day smoking habit continued.
"I felt fine," says Derek, who is married with two grown-up children. "There was no impact on my health, so I saw no reason to change my ways."
He received his first warning when he was diagnosed with high blood pressure in his late 50s. His GP advised him to quit smoking and adopt a healthier lifestyle.
But Derek thought that if he took his medication for high blood pressure and changed his diet, he could go on as usual. "I never felt ill and the high blood pressure didn't have any real impact on me, so I continued with my lifestyle," he says.
"The fact is, I never thought I could quit," he says. "Although I thought about attending a smoking clinic, the sessions were never held at a convenient time for me. In hindsight, I realise I was just making excuses."
Derek remembers feeling a dull pain in his chest that started on Christmas Day at home in 2005. It continued on and off into Boxing Day.
"It was nothing you could call a heavy pain, not the way heart attacks are usually described, but it just went on and on."
Unable to sleep, he got out of bed. His daughter found him sitting at the top of the stairs. She knew something was wrong and called an ambulance.
Tests carried out at hospital confirmed Derek had suffered a heart attack. "It was a bolt out of the blue," he says.
All these years, Derek had been working for Heart Research UK, preaching the ills of smoking, while he himself continued to smoke.
He had ignored the warnings: his high blood pressure, his age (which placed him in a higher risk category for heart disease) and his GP's advice.
"I was hiding from the truth," he says. "I just didn't think it would happen to me."
At the hospital, Derek had an angiogram, which measures blood pressure inside the heart, to find the cause of the heart attack.
He went on to have an angioplasty, which is an operation to open up blocked or narrowed blood vessels leading to the heart. He had three stents implanted to make sure the narrowed blood vessels remained open.
While on the cardiac ward, two events made a lasting impression on Derek.
He was awoken one night by the sound of medics resuscitating a man without success, and on another occasion he saw porters wheeling out a coffin.
The experience scared him to death, he says. "After that, any thoughts of smoking again completely vanished."
He's since made a full recovery. "I'm pleased to say I've not touched a cigarette since," he says.
Derek is a lot more careful about the food he eats. He makes sure he has a varied diet and eats five portions of fruit and vegetables a day.
He keeps active by taking his granddaughter's springer spaniel for brisk daily 30-minute walks, whatever the weather.
He says: "The heart attack left me feeling fragile and vulnerable, because I came so close to the end. It was a wake-up call, but not the type I would have chosen."