The 33-year-old has been working out at Barry's Boot Camp in New York City, reported Showbizspy.
"Katie has been going to Barry's Boot Camp in New York City and she is working really hard. She looked great, well rested and seemed happy," a source said.
"She was dressed down and looked casual and comfy. Her hair was wavy and natural. During class, Katie was holding her own on the treadmill and was a fast sprinter! She was super friendly with the other girls next to her.
"One mentioned how she is the coolest, most down-to-earth celebrity she has ever met," the source added.