Michael Phelps' swimming comeback hits another speedbump

Michael Phelps was back in the pool at the U.S. National Championships on Wednesday. But after finishing in seventh place, it became clear that his comeback won't be easy, CBS News correspondent Michelle Miller reports.

Wednesday's 100-meter free final was the Olympic champion's biggest race since the 2012 Games.

Phelps barely touched the wall at the halfway point, losing all momentum and costing him the race.

"There was very little chance that I was gonna run anybody down, so it's kind of frustrating, but I'll never really know where we are in that race right now," he said.

Current Olympic champion Nathan Adrian finished first and Phelps' longtime rival, Ryan Lochte, placed second.

"I was like 'What happened?' and he said he missed his turn, but you know, things happen," Lochte said. "I guarantee he's going to fix it and make sure it never happens again."

It's been three months since the 29-year-old champion came out of retirement, committing himself to the 2016 Olympics in Rio.

The most decorated Olympian in history, Phelps' comeback has been far less stellar.

"For all of his superhuman feats, he's actually human," said New York Times sports writer Karen Crouse. "And maybe this race reminds us all that he's mortal, there's something good to come out of it."

In April, Phelps' dream of a flawless return was dashed with a second place finish to Lochte at the Mesa Grand Prix in Arizona. But even without a spot on Wednesday's podium, Phelps says his best is yet to come.

"It doesn't really matter to me about making a comeback or not," he said. "I'm very hard on myself no matter what, it's just kind of frustrating to not start out with a good one."