Loping down Commonwealth Avenue Monday afternoon in a sea of fellow runners, I peered ahead toward the long-awaited right turn onto Hereford Street, the last intersection before the final hook onto Boylston Street and the finish line of the 117th Boston Marathon.

"Less than half a mile to go," I told myself. "Stay focused."

Suddenly, a knot of shorts-clad men and women milled about in the middle of the street, blocking my path.

"Hey! Do you mind getting out of the way?" I cried, or words to that effect.

They ignored me and I had to swerve sharply to avoid a collision – something the legs don't enjoy doing after they've been running steadily for nearly 26 miles over the past four and a half hours.

Then I could see the entire roadway was clogged with stationary marathoners. What the heck was going on?

"What are you guys doing?" I asked, grinding to a halt.

"There's been some kind of explosion at the finish," somebody on a cellphone reported.

The hairs on the back of my neck stood on end.

"Anybody hurt?"

"Hard to say. It just happened."

Another runner who had been ahead and then forced to reverse direction had a first-hand report.

"I heard two blasts, one right after the other."

"Oh my God!"

College kids, spectators with smart phones and apartment-dwellers who had been following the news came out to fill us in on the horrible details, and to offer us water, snacks and garbage bags to wear as windbreakers against a chilly breeze while we shivered.

Just ahead I saw my buddy, Phil Plouffe of Mystic, with whom I'd been running side-by-side since the starting line in Hopkinton until he pulled slightly ahead going up Heartbreak Hill at Mile 18 when I made a short detour to a Porta-Potty.

"Hey, Phil! What's going on?"

He muttered something unprintable.

Phil had wanted to run the Boston Marathon for as long as I've known him, more than 30 years, when we met while training for our first long-distance race, the East Lyme Marathon. Over the years we've hiked, mountain climbed and kayaked together extensively. Monday was to be his first Boston, and it was to be my 10th and last.

I thought 10 was a good, round number to end on, since it would coincide with the first for my son, Tom.

By dint of a speedy qualifying time of 2:45 at the Denver Marathon last year, Tom was given a low starting number, 685 in a field of some 28,000, while his old dad had number 20,458 and had to wait 40 minutes for the staggered start to begin.

Tom finished in a personal-best time of 2:42:20 and had already showered and changed at a friend's apartment when the bombs exploded, I learned in a relieved phone call to my wife, Lisa, using a borrowed cellphone while we runners huddled so close to the finish line.

After more than a half hour of confusion and chaos, compounded by hundreds of emergency vehicles roaring past, sirens blaring, a Boston Athletic Association official shouted above the din that we should continue on foot on Commonwealth Avenue, but instead of turning onto Hereford we should walk another mile or so all the way to Arlington Street near the Boston Common, where buses with dry clothes and other gear we loaded in Hopkinton would be waiting.

Police barricades and emergency vehicles blocked every intersection leading to the finish line.

The cheering throngs of spectators that normally greet finishing marathoners either disappeared or, like us runners, shuffled away ashen-faced. Some sobbed.

At this point I certainly don't care much who won the race, and care even less if I ever do earn that 10th finishing medal. Monday's tragic murders and maimings overshadow any such considerations.

As of this writing authorities don't know who is responsible or why, so it's pointless to speculate on motives or to pontificate other than to say we are shocked, angry and horrified.

The Boston Marathon has always been a joyous event, even for those who have struggled to cross the finish line.

The deafening exuberance of the crowds, the non-stop cheering from Hopkinton to Boston, the live bands, the front-porch parties, the kids begging for high-fives and handing out cups of water and orange slices – we'd hate to all see that lost.

At the same time, the victims of Monday's bombs are no different from those at a school or a shopping center or a movie theater. They all died or were injured by incomprehensible acts.

We mourn them all.