Here’s an encouraging sign for Daniel Kilgore: He’s more worried about his lungs than his left leg.

On Monday, the 49ers center said he’s not concerned about his surgically repaired leg withstanding the strain of blocking industrial-size nose guards. He is, however, worried about getting winded 13 months after he played in his last NFL game.

“The biggest thing for me would be the conditioning part,” Kilgore said. “My ankle and leg feel stable. Now I have to go out there and test it out and go see how out of shape I really am.”

Kilgore offered that with a smile. And he had reason to be happy. On Monday afternoon he took part in a walkthrough, his first practice since his injury, after the 49ers opened his three-week practice window earlier in the day.

Kilgore remains on the physically unable to perform list, but the 49ers can place him on the 53-man roster anytime in the next three weeks. Kilgore said he’d like to play Sunday against the Seahawks, but acknowledged the 49ers’ game against the Cardinals on Nov. 29 was a more realistic goal.

Kilgore said he’s back to squat-lifting 500 pounds and has had no issues slamming into blocking sleds during his recent rehabilitation work. He’s confident in his left leg, which has required two surgeries and now has a plate and 12 screws in it. However, he acknowledged his first test won’t arrive until he makes his 2015 debut.

“I can do as much as I can against a dummy or a pad,” Kilgore said. “But until I go against somebody that’s breathing and trying to kill me, we’ll see then.”

Even if he’s rusty, Kilgore figures to provide a boost for an offensive line that’s struggled in his absence. His replacement, Marcus Martin, 21, a 2014 third-round pick, has been consistently bullied in the run game and has ranked among the bottom of the NFL’s centers throughout the season, according to Pro Football Focus’ grading system.

The 49ers have allowed the fifth-most sacks in the NFL (28) and their running backs are averaging 3.5 yards a carry.

The issues up front have inspired experimentation. Starting right guard Jordan Devey has split snaps with Andrew Tiller in the past five games, and it’s possible Tiller will make his first career start in Seattle on Sunday. In a 17-16 win over Atlanta on Nov. 8, Tiller came off the bench to play 48 of 57 offensive snaps. In addition, rookie right tackle Trent Brown made his NFL debut in a nine-play series against Atlanta while starter Erik Pears moved inside to right guard.

“I’ve said it with the offensive line: It’s the sum of the parts,” head coach Jim Tomsula said. “We’re getting the guys together and working together to see where we can get the best results.”

For his part, Kilgore is eager to see results at practice this week, where he’ll renew on-field acquaintances with nose guard Ian Williams.

“He and I have always had a friendly, competitive spirit going against each other,“ Kilgore said. “It’s just going to be fun going out there again. Being with the guys, being in the huddle. The only thing I’m not looking forward to is wearing the helmet. That thing is going to hurt like crazy. Been a year since I’ve worn it.”

Briefly: Tomsula said running back Carlos Hyde (foot), wide receiver Anquan Boldin (hamstring) and cornerback Tramaine Brock (shin) have yet to run at full speed and remain day to day with their injuries. Hyde and Boldin have missed the past two games, while Brock has missed one. … Rookie wide receiver DeAndre Smelter, who is on the non-football injury list after he tore his ACL in November, also had his three-week practice window opened. … Tomsula reiterated that quarterback Blaine Gabbert would start against Seattle, but he wouldn’t commit to Gabbert making another start after Sunday.

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com Twitter: @Eric_Branch