09/23/2012 AT 09:05 AM EDT
Joking with others, shooting hoops and scoring baskets – to look at Scott Peterson, one would hardly imagine he's incarcerated on San Quentin's Death Row.
And yet, that is how journalist Nancy Mullane found the former fertilizer salesman when she visited the California state penitentiary where Peterson is confined to a solitary 7-by-9-ft. cell for the Christmas Eve 2002 killings of his wife Laci
and their unborn son, and then dumping their remains into San Francisco Bay.
"He didn’t look depressed. He looked like someone you'd see on the street playing basketball. He had his shirt off and his boxer shorts up," says Mullane, who was given an unprecedented level of media access to the prison for her book about the rehabilitation of condemned killers, Life After Murder: Five Men In Search of Redemption
. "He wasn't ripped, but he looked healthy."
She says Peterson, who turns 40 next month, may have put on a couple of pounds but didn't look much different from when he was sentenced in 2005. As for his basketball game, which had the inmate's complete interest, "He was playing a pretty hard game," says Mullane. "There was a lot of jumping around and making baskets" as guards armed with rifles watched from nearby.
After the game, she says, Peterson returned to his cell wearing a baseball cap and sunglasses. A few minutes later, Mullane saw him walking to the shower in flip-flops and shorts. Mullane also noted that Peterson was one of the youngest men in his cellblock.
Sources close to Peterson tell PEOPLE he'd been intensely focused on a 423-page appeal that his lawyer would file three weeks after Mullane's observations. They say he has had legal papers and books stacked in his cell amid high hopes he'll prevail.
The appeal, which was filed on July 5, could take months or even years to resolve. (All death-penalty sentences receive mandatory appeals in California.)
"While most people on Death Row are guilty and want to therefore slow down the legal process, Scott is trying to go full speed ahead," Pat MacGuire of Ohio, a regular Peterson pen pal, tells PEOPLE. "He wants to get the person who killed Laci."
MacGuire adds that Peterson avoids eating the burritos, candy bars and other unhealthy foods from the prison cantina (even though the prison meals are bland), because "Scott believes someday he'll be set free and he doesn't want to be diabetic when that happens … he says he'd rather eat chopped-up broccoli stalks than a bag of Doritos."
Peterson also practices yoga and reads Westerns and other books to pass the time, says a former defense attorney, Matt Dalton.