In high school, Kastor won three California state cross country titles and two CIF California State Meet titles at 3200 meters while running for Agoura High School in Agoura Hills, California. She also competed in the Foot Locker Cross Country Championships all four years of her prep career, and competed in both the North American YouthMaccabi Games and the Pan-American Maccabiah while in high school. At the University of Arkansas she was a four-time SEC champion and an eight-time All-American. Post-collegiately, Kastor ran under coaches Joe Vigil and Terrence Mahon. Since 2015, she has been coached by her husband, Andrew Kastor head of the Mammoth Track Club. Kastor has earned two silver medals in the IAAF World Cross Country Championships. She holds U.S. records in the following events:
In April 2008, Kastor won the U.S. women's Olympic marathon trials in Boston, Massachusetts. She finished with an unofficial time of 2:29:35, after overtaking competitor Magdalena Lewy Boulet in mile 23. Kastor ran most of the race from behind, while Lewy Boulet built a commanding lead very early on, running alone for most of the marathon. With some to go, Kastor made a move to catch up to Lewy Boulet, stringing out the field. Lewy Boulet took second place in 2:30:19. In August 2008, Kastor pulled out of the women's marathon at the Beijing Olympics with a foot injury. At about the mark, she dropped to one knee, holding her right foot. She attempted to rise, but dropped back down again and was forced to withdraw from the race.
2010
On March 21, 2010, Kastor competed in the first spring running of the NYC Half-Marathon. After running the majority of the race in first, on her way to breaking the course record, she dropped to second place to finish behind Great Britain's Mara Yamauchi. It was announced in August 2010 that Kastor and her husband were expecting their first child, Piper Bloom, in March 2011. It was also announced that Deena would be making her return to racing at the New York Mini 10K.
2012
In January 2012, Deena ran 2:30:40 to place 6th at the Olympic Squad Houston Olympic Trials.
2013
In January 2013, Kastor announced she would be running in the 2013 Los Angeles Marathon, to be held on March 17, 2013 where she finished third in 2:32:39. On August 10, 2013, Kastor placed 9th at the World Championship Marathon in Moscow with a time of 2:36. She stated that it may have been her last high-level marathon.
2014
In April 2014, the 41-year-old Kastor won the 2014 More|Fitness Half-Marathon in New York's Central Park in a U.S. masters record of 1:11:38. On September 21, 2014, she set the world record in the Women's Masters division for the half-marathon, at 1:09:39, while running in the Rock 'n' Roll Half-Marathon in Philadelphia.
2015
In October 2015, she broke the U.S. Women's Masters marathon record by almost a minute at the 2015 Chicago Marathon, running 2:27:47.
Awards and rankings
Kastor was selected as the top women's marathoner in the world in 2006 by Track and Field News magazine. Among the honors Kastor has received from the USATF are:
