"It went really well. I'm really happy," Walker said. "My arm felt great, and I was throwing about 85-90 percent. It was really coming out good, I thought."
The 21-year-old hopped off the mound with a small fist pump following his final throw, looking like a youngster who'd just aced an important test.
"I was pumped," he said. "I know location doesn't matter and it's about how my arm feels, but my arm felt really good and the location I was spotting up pretty good. So I was real happy with that."
After Walker's initial bullpen session, a lighter 25-pitch outing on Thursday, he said he felt like he was starting Spring Training over. But he's getting back into a normal routine now, although he's a month behind teammates who've been throwing since mid-February.
"Today I felt like it was probably my 10th bullpen. That's how good I felt," he said.
Walker threw just fastballs and changeups, with about 15 warmup tosses before he started airing it out.
"I popped the last 15 or so and was really letting it go," he said. "There was no soreness. Nothing. I feel really good."
It's been a long camp so far for the youngster, who came to Peoria gunning for a spot in the rotation after going 1-0 with a 3.60 ERA in three late-season starts last year. But he overthrew prior to camp and arrived with a sore shoulder that never went away.
The Mariners eventually flew him to Los Angeles for tests, and he was shut down for a week after being diagnosed with bursitis, or inflammation in the shoulder. With no structural damage, the club felt the seven-day rest would allow him to heal up and start fresh.
So far, that has been the case. Now Walker just needs to move beyond worrying about his wing every time he wakes up or steps on the mound.
"When I throw, the first couple I think, 'I hope I don't feel it,'" he said. "Then I don't feel it, so I don't think about it. But maybe in a week or two I'll just wake up and go to the field and play catch and never worry about it at all."
Walker, the Mariners' top prospect and rated the No. 6 prospect in baseball this spring by MLB.com, says he's not certain on his next step. He knows patience is still required, which isn't easy when you're looking to crack a Major League rotation at any age.
"Hopefully I throw live BP or another bullpen next," he said. "I'm not sure. I'm just taking it day by day. I'd like to get in a game, that would be nice. But yeah, probably another bullpen or something like that in the next couple days."
The Mariners could use some good news in the pitching department, as three starting candidates -- Walker, Hisashi Iwakuma and Brandon Maurer -- will not be ready by Opening Day due to health issues.
Iwakuma, who was third in last year's American League Cy Young Award voting, has yet to throw this spring. He's been dealing with a sprained tendon in his right middle finger, injured when he caught it in netting during a pitching drill just prior to camp. Iwakuma is set to visit a hand specialist in Phoenix on Tuesday, hoping to get word that he can finally take the splint off his finger and start pitching again.
But as with Walker, Iwakuma will need to throw several bullpens, then work up to live batting practice and simulated games before stepping on the mound in a real game situation. And the Mariners will likely have both throw a couple Minor League outings to build their innings and pitch count before they make their Major League returns.
In a best-case scenario, that puts both pitchers back in mid- to late-April, having missed several turns through the rotation.
Maurer is sidelined with a bad back and has only thrown in two games this spring. That leaves a wide-open rotation competition behind ace Felix Hernandez, who is on schedule to make his club-record seventh Opening Day start on March 31 in Anaheim.
Erasmo Ramirez, James Paxton and Blake Beavan are competing for the other four openings, with non-roster veterans Scott Baker and Randy Wolf also in the mix. And young lefty Roenis Elias, who pitched Double-A ball last year, is getting a good look now as well.
But the rotation that starts the year could change quickly, once Walker and Iwakuma return. And Walker liked how things felt Sunday in his latest test.
"I really wasn't working on location, I was just trying to get a feel for it," he said. "But I felt really good, so I just figured I might as well take the next step and I was spotting up pretty good. So I was, 'OK, this is good. This is a good sign.'"
And the Mariners will take that.