"I just thought my hair had had a life and I wanted to kind of put that behind me and start fresh," the actress told Ellen DeGeneres. "And I thought it would be good for the marathon. It was!"
Anderson, 46, joked that she "stole" the look from DeGeneres, who previously told Anderson that she'd never complete the marathon. (She did, and raised money for Haiti in the process.)
"It was hard," Anderson said. "The last five miles were like giving birth. It was like giving birth and being told to run as you're giving birth."
To recuperate, the former "Baywatch" star, whose said her hips were killing her, took an ice bath - and made plans to do it again next year.
"My oldest son, Brandon, told me next year he'd do it with me!" she said.