Biden said he needs to "pace himself" when pressed on how he is up to the 24/7 nature of the presidency -- while taking a crack at Trump's schedule compared to his own.

"Where has Trump been? Riding on his golf cart and filling out his scorecard?" Biden said. "He has done virtually nothing. I've had roughly 20 major events, some with thousands of people showing up."

Biden said he has always had an inclination to "keep going" but "I just have to pace myself a little more."

"In the next debate, I'm not going to be traveling in 15 time zones the week before," he said.

Biden addressed his previous statements that Vice President Kamala Harris "would be ready on Day One."

The president touted Harris' work with women's reproductive health issues and her time in the Senate.

"I wouldn't have picked her unless I thought she was qualified to be president. From the very beginning, I made no bones about that. She is qualified to be president. That's why I picked her," he said.

Biden was asked to address his gaffe earlier Thursday in which he introduced Ukrainian President Volodymyr Zelenskyy at a meeting about the Ukraine Compact at the NATO summit as Russia's President Putin, and if other world leaders needing to step in and make excuses is "damaging" to America's standing in the world.

"Do you see any damage by me leading this conference?" Biden responded. "Have you seen a more successful conference? I was talking about Putin and at the very end I said, I'm sorry, Zelenskyy."

"I thought it was the most successful conference I've attended in a long time," he later said.

One reporter asked the president, "Have you spent time thinking about what it would mean for your legacy, which you've worked decades to build, if you stay in the race, despite the concerns that voters say they have, and you lose to someone who yourself have argued is unfit to return to the Oval Office?"

Biden replied, "I’m not in this for my legacy. I’m in this to complete the job I started."