Doctors discovered Johnston was suffering from a brain tumor.
Because the tumor was in the base of his brain, doctors were unable to remove it through surgery.
The 42-year-old Johnston, who retired from the Charles Town Police Department after his diagnosis, said he was told he had between three and six months to live.
"Pretty much I was given a death sentence," Johnston said.
Things were rugged in the times ahead.
Johnston said he became weak, and when he would try to stand up sometimes, he would collapse on the floor.
"I was pretty much bed-ridden," Johnston said. "It was to the point I couldn't throw a football to my kids. It was so hard to accept."
At one point, Johnston weighed 305 pounds and was able to squat 700 pounds in weightlifting.
Johnston recalled one of the tougher moments during his illness when fluid began to build up in his brain. Johnston said the excess fluid caused him to have blackouts, and he was flown from Jefferson Memorial Hospital in Ranson, W.Va., to Johns Hopkins University in Baltimore, where he was being treated.
"I remember waking up in the hospital," he said.
In their attack on the tumor, doctors administered radiation and chemotherapy to Johnston, and the tumor shrank.
Johnston previously said that doctors extended his life expectancy by two years.
When Johnston was being honored by Charles Town City Council members in 2006, he said hopefully his life expectancy will be extended to five years the next time he sees the doctors.
Whatever the reason, Johnston is making a comeback.
For starters, the tumor now is considered inactive, Johnston said.
And he's mobile.
From the bed, Johnston moved to a walker to get around, then to a cane.
Both are gone as Johnston -- known for his hulking size -- walks confidently.
The steroids that used to cause Johnston to have what he called a swollen "moon face" appearance are gone from his treatment and his skin has a pink, healthy appearance.
The days of old, when the muscle-bound Johnston walked around town in intimidating fashion, are coming clear again as tattoos on his large arms peek down from beneath a shirt with the sleeves rolled high on his arms.
A black necklace holding a sliver medallion is tight on his neck, and his goatee and bald head accentuate his presence. And he talks with conviction about his recovery and how God, his wife, and his family and friends have been integral in bringing him this far.
"I tell you, it's a miracle," Johnston said.
Johnston now can add another accomplishment to his survival -- a return to law enforcement.
Three weeks ago, Johnston started working part time at the Jefferson County Sheriff's Department to help run the home confinement program and do background checks on people seeking concealed weapons permits.
Johnston said he was ready to get out of the house, and Jefferson County Sheriff Everett "Ed" Boober agreed to bring Johnston onto the department.
"I feel like I'm back in my element," said Johnston, whose police career involved undercover work.
As far as his life expectancy, Johnston said doctors are telling him to take it one day at a time.
"The highest doctors can't explain it," Johnston said of his progress.
Robert E. Jones, an internal medicine and pediatric physician in Ranson is Johnston's primary care doctor. Jones said he made the initial diagnosis on Johnston's tumor, and Johnston later was referred to Johns Hopkins University for treatment.
Jones said no one gave Johnston much of a chance of survival and they lost hope. But one doctor at Johns Hopkins University kept working with Johnston, Jones said.
"They can't believe it either," Jones said of the doctors at Johns Hopkins. "I think it falls just short of a miracle."
A slight limp on Johnston's right side when he walks and a "shunt" that is visible under the skin of his scalp are the few visible signs of Johnston's battle with the tumor.
A shunt is a hole or passage, and Johnston said it was placed under his skin as part of a tube that extends into his stomach to drain any fluid off his brain.
Johnston said he still has some weakness on his right side, and because as a police officer he knows the dangers of painkillers -- especially Oxycodone -- he weaned himself from the powerful drug. And he's back in the gym for workouts to build strength.
Johnston's friends and family members are thankful for his improving condition and presence.
When Johnston expressed an interest in coming to the sheriff's department, everyone was "immensely interested," Lt. Dave Colbert said.
"We're all just ecstatic," Colbert said.
"He was supposed to be dead two years ago," said Ranson Police Department Sgt. Todd Lutman, who considers Johnston to be his best friend. "Every day's a blessing. He's one of the lucky ones so far."