Don Callahan knows the Boston Marathon. From its storied hills and slopes, to its glorious winners and notorious cheaters, Callahan has been a spectator of the world’s most popular road race 47 of his 51 years.

The Wayland native and avid runner knows everything about the Marathon — except the feeling of crossing the finish line with thousands of others after 26.2 of the state’s most famous miles.

That will change Monday.

In a courageous effort to raise money for Scott Carlson, a friend of 15 years suffering from Lou Gehrig’s Disease, Callahan will run his first marathon in 22 years Monday in the name of the Angel Fund — a Massachusetts General Hospital program still searching for a cure.

" I wanted to do my part to help continue the research, " Callahan said. " And it gives Scott a chance to stay involved in something important. "

It took Callahan a while to get involved himself. Last year, Carlson urged him to take part in the cause.

" But I told him that training for 26.2 miles during a New England winter wasn’t exactly appealing to me. "

Callahan finally came around this year.

" After a year I saw what it meant to him and it was an easy decision, " he said.

On Christmas morning, he made it official.

" It was about 5 degrees that morning, and I woke up for a run with two friends, " he recalled. " I pretty much made up my mind, but I just wanted to get their feedback and ask, ‘Hey, can I do this?’ "

Since then, it’s been about 50 miles a week for him through Wayland and Sudbury, and countless callouses and cramps along the way.

" I’m definitely tired of running slow and preparing for this, " said Callahan. " I’m a triathlete, but 26 miles is a long way for anybody. "

But seeing the condition of his friend and his helpless state has pushed him each day.

Typically patients with Lou Gehrig’s Disease live 3-5 years after detection. Carlson was diagnosed two years ago.

" It’s a horrid disease, " Callahan said. " You see people waste away. You lose the ability of your muscles and central nervous system. It’s a progressive disease that causes atrophy.

" I wanted to do everything I could do to help. "

Realistically, Callahan is doing something he should have done years ago. A lifelong runner and Boston Marathon expert, Callahan saw the glory first-hand at age 5 when his parents took him to the finish line.

He’s been hooked ever since. From then on, his recreational life has been centered around running. He spent years as the president of the Greater Boston Track Club, was a Marathon commentator on WRKO radio, and was once a finalist for the Marathon’s Race Director position.

" I’m feeling a lot of different emotions leading up to this, " he said. " I’m anxious for it to come, I’m frightened of the 26.2 miles and I’m thrilled to get the chance to do this. "

Of course, there will be other feelings outside of ecstasy after the race is finished.

" It’s going to hurt, " he laughed. " Really bad. "

But it’s a small price to pay to join the most elite running fraternity in the world.

" What other sporting event does a regular guy get to compete with the world’s greatest in that sport, " he asked. " Mere mortals can race against Olympic champions. "

While the majority of his training has been completed, he now finds himself in the resting period these final days.

" I’ve gone over the course a few times, just to be ready, " he said.

He said the route has started to get packed with Marathon entrants just as curious as him.

" I’m excited to see Wellesley College, " Callahan said. " That’s a big one for me. I can’t wait to see what all the Wellesley College students are shouting. "

And he said he may stop to soak it all in.

" I’m not going for some record time, I want to enjoy the experience, " he said. " If I see an old friend, I may just stop to say hello. "

Of course, he’ll be running alongside about 24 friends who are all giving 26.2 miles in the name of the Angel Fund.

" We’ll be out in force. "

As will be Carlson, a Rhode Island resident, who has promised to be the first person Callahan sees at the finish line.

" I didn’t give him much of a choice, " Callahan said. " I told him he’d better be there. "