Williams, who qualified for the Boston Marathon before learning she would be at the international space station on race day, was cleared for a 26.2-mile simulated run at 10 a.m. today - just when the real event will be leaving Hopkinton down on Earth.
That will allow the U.S. Navy commander to run at the same time as her sister, Dina Pandya, fellow astronaut Karen Nyberg and about 24,000 others who are expected to face heavy rain and head winds on their way to Boston's Back Bay.
Williams will run the equivalent distance at the space station, in low orbit about 210 miles above Earth, while tethered to a treadmill by bungee cords so she doesn't float away. In the past, the space station's treadmill has had its share of mechanical problems.
"I think both of us are as ready as we're going to be," Williams told The Associated Press, referring to the treadmill.
Williams qualified for the race by finishing last January's Houston Marathon in 3 hours, 29 minutes, 57 seconds. But she left on the shuttle Discovery on Dec. 9 and has spent the past five months in space.
Because of the demands of Williams' sleep and work schedule, NASA had considered having her run Sunday evening. But spokeswoman Eldora Valentine confirmed on Friday that Williams will be able to run at the same time as the main event.