After failed relationships with Matthew McConaughey and Tate Donovan, Sandra Bullock says she's through with dating her co-stars.

"You're thrown into a situation where you have to create this artificial intimacy with the guy you're co-starring with," the 28 Days star tells McCall's. "It's unnatural because you spend more time with this person than you would with your significant other. In the beginning you think, 'Oh, he's so sweet and attentive.' Then you realize it says he has to be that way in the script."

Although McConaughey dumped her after two years, the actress has no hard feelings toward her A Time to Kill co-star. "Of course it's more fascinating to have a horrible breakup where you end up hating each other, but that's just not the case here," she claims. "We consider ourselves good friends."

Bullock is still looking for love and says she's finally feeling prepared to have children. "It's taken me a long time to be comfortable in my own skin. I just didn't want to subject my kids to a neurotic mother. I always wanted to feel as if I could put myself first. I'm very close to feeling that way now."

The death of Bullock's mother, Helga, may have prompted the star to make a few changes. "My mother used to be a marathon runner when she was younger, and I've always wanted to do this, too," she explains. "I feel like I owe it to her. I'm training now, and it's addictive. I'm starting with the Ks first ? like the 10K, which is six miles ? and moving on up from there. There's just no stopping me."