Deena Kastor has become a mentor, friend and training partner to Milton’s Kate O’Neill, and the relationship has benefited both Olympic marathon aspirants.

Kastor and O’Neill have been training together for months under the direction of Terrence Mahon in Mammoth Lakes, Calif. But they will embark on separate missions in tomorrow morning’s U.S. Olympic Trials. The race begins and ends on Boylston Street, and three spots are available for the Beijing Games.

“She’s been a wonderful training partner and we’ve been training together for quite a while,” Kastor said yesterday at the Fairmont Copley Plaza Hotel. “We have been preparing for the same distance, and the experience has benefited both of us. She’s so strong and spirited. She’s brought the best out in me, and I feel she has grown stronger herself. I’m very grateful having her as a teammate because you need someone who will go the distance with you on a daily basis.”