At age 54, Marshall has been diagnosed with Lou Gehrig's disease. His muscles are being destroyed, and eventually his lungs will shut down.
His fingers already have started to fail him and Debi, his wife, helps him dress, CBS News correspondent Jerry Bowen reports.
In one of life's cruel twists, Debi Marshall knows about life-ending illnesses.
"Fourteen years ago I was given days, if not hours, to live," she says. "I had five kids under 10, and I was 37."
It was advanced non-Hodgkins lymphoma — an incurable cancer. Debi started planning for her family's life after she was gone.
"We had this expectation that eventually that she would probably die before I did," Bob says.
"I even went so far as to pick his wives. You can't marry this one; you can marry this one," Adds Debi.
Year after year, holidays, birthdays and vacations were treated as Debi's last. The children learned to take life a day at a time. But it was hard.
"You get home from school and your dad rushes down, 'hey you guys might want to say goodbye to mom because she is not doing too well,''" said Debi and Bob's daughter Tiffany.
But 30-plus surgeries later and 64 chemotherapy sessions and counting, her life goes on. Against all odds, she's survived.
Then came that day last October, shortly after Bob had run a marathon in Utah, that their world turned upside down. After all the years of planning for Debi's death, doctors said Lou Gehrig's disease might take Bob first.
Suddenly, the Marshall family rock — Mr. Mom — was in trouble.
"Everything was always going to be OK as long as he was around," Tiffany says. "He took care of pretty much everything."
So plans changed. Debi is trying to learn the ropes of the family's small newspaper publishing business.
But plans for Boston stayed on course. And trailing the army of drenched runners was Bob Marshall. Bent by exhaustion and pain, the man who can't tie his own shoes was determined to make it.
When he finally crossed the finish line, six hours later, there was no shortage of tears — from his teammates and from his family. Everyone knows his time is running out. But on this day, he never felt more alive.