You can search back 'till 1979, so I merely searched on "John Kerry" marathon.
I received 50-some odd hits and paid to view up to 3 full articles (excerpts appear in the basic search).
Since we may only post excerpts here, I'll post the three articles I did view for those who care to pick up the standard and check out others in search (vainly, I fear) of any definitive "John Kerry ran the Boston Marathon" reports in past editions of the Globe.
#1-"Sports" section item, by Dan Shaughnessy, April 20, 1982
"Marshall Medoff was in a good mood when the 86th Boston Marathon wound down in the Prudential garage yesterday afternoon. He spent a good part of the post-race rush hour wading through an ocean of exhausted runners, interviewing competitors and slurping from a cup of yogurt - one of 12,500 donated by the Columbo Co. of Methuen..."
KERRY CONTENT (paraphrase): Then-Lt. Gov candidate Kerry had a plane fly an aerial ad over the race course/finish line area.
SHORT CIRCUITS Author(s): Reported by Globe staff writers. Date: April 27, 1986 Page: A29 Section: FOCUS
"As the Boston Marathon prepared to get under way last week, US Sen. John Kerry, who was to serve as starter for the wheelchair division, took an exit off Interstate 495 that was closed to all but official vehicles..."
SUMMARY: Kerry picked up the racers and took them to the wheelchair starting point.
THANKS TO MCGILLIVRAY, A FINE START Author(s): Michael Vega, Globe Staff Date: April 19, 1988 Page: 56 Section: SPORTS
"HOPKINTON -- There were no pratfalls -- not anything resembling a single slip-up -- at the start of yesterday's 92d Boston Marathon..."
KERRY CONTENT: Kerry was the official starter for the wheelchair race, a ceremonial position he seems to have held more than once in the past.
Odd, too, that he doesn't mention the details of his involvement with the race. One would think his involvement with wheelchair-bound participants in the past is something he'd at least mention in passing when discussing his past involvement with the race.
WOW. you have almost-successfully eliminated the period from 1979-forward, which is significant.
It also has the side benefit of showing that there is no record of him having run a qualifying marathon (sub-2:50 or sub-3:00, depending on the year), at least in the area. Of course, he could have done one anywhere else in the country, but even then there's a chance the Globe would have picked it up and noted it, especially since Kerry's the type to have wanted the acheivement of having qualified reported.