Word to the wise: Maybe it’s not the smartest thing to jog in what looks like a gas mask and body armor, and then jam a package in a post office box.

It could touch off what happened Tuesday at the busy San Jose post office on Lundy Avenue: a full-scale police response, complete with the bomb squad and a robot.

But San Jose police said the suspicious-looking jogger seen fidgeting with a package at a drop-off box was only working out in hard-core, albeit odd-looking, exercise gear.

“The guy said he was wearing a cardio mask,” said Sgt. Jason Dwyer. “It was his cardio day, and he was trying to lose weight.”

That’s not what a post office customer thought when the man in the weird mask and vest was stuffing a package in a blue mail box about 12:30 p.m. The customer called police, and in a flash, the post office was on lockdown until 4:30 p.m., with 150 employees and customers tucked away in the back. The San Jose police bomb squad, the Fire Department’s hazardous materials unit and the postal inspector descended on a normally quiet strip of North San Jose. A robot detonated the package, which turned out, police said, to be a bunch of calendars.

“My friends kept messaging me, Is this you? Is this you?” recounted Long Hoang, who lives near the post office and is a student at Cal State East Bay in Hayward.

It was.

About 12 weeks ago, Hoang became an avid follower of the CrossFit exercise regimen, which he said, exuberantly, combines “this really creative combination of weight lifting, gymnastics and rowing.” He wears the mask to simulate high-altitude training. Hoang, who is 5 feet 4 inches, said he’s lost at least 20 pounds, and is now 142 pounds.

Many neighbors in the area frequently spot him running in his gear, doing squats and lunges at corners while he waits for the light to turn green.

As Hoang tells the story, he mistakenly received a package of calendars at his home, and thought he’d mail them back to the proper recipient while on an exercise run. The package didn’t fit the first time in the mail box, he said, so he had to fold it up and try a second time.

When Hoang later realized his actions had caused such a commotion, captured by this newspaper and many other media organizations, he called police Tuesday evening to say the suspicious man was him: a nursing student trying to get into shape.

Police said Hoang’s story checked out.

Hoang said he can’t really see what all the fuss was about.

“It was like straight out of a movie,” he said. “Some of my friends are telling me, ‘Hello? 9/11? Anthrax? Blah. Blah. Blah.’ And I’m just thinking about my finals and staying in my own little zone.”

Still, Hoang says he won’t be jogging to the post office any time soon.

Contact Lisa Fernandez at 408-920-5002.