DALLAS - Preparing for what may be the biggest physical challenge of her life, Shakar Mirza got into shape by eating healthy food, walking every day and attending weekly aerobics and yoga classes.

The event? The hajj -- the annual pilgrimage that draws millions of Muslims to Mecca.

Mirza, 68, of Carrollton, Texas, left last week for her second hajj. While the journey has profound spiritual significance, it's also physically taxing, especially for older Muslims. Many pilgrims, like Mirza, have to cross half a world just to get to Saudi Arabia. Then they travel from city to city, at times sleeping in tents or under the stars, with temperatures as high as 110 degrees.

As an Islamic Web site, http://www.islamicity.com/, cautions: "The hajj rites require a great deal of strength and endurance. The constant crush of hundreds of thousands of pilgrims, each trying to perform the same rites at the same time in limited spaces and very hot weather, compounds the demands on your physical conditioning and mental toughness."

Having made the trip four years ago, Mirza knew what to expect.

"I'll be there 21/2 weeks, and the whole time, it never lets up," she said. At one point in the pilgrimage -- when she will walk around the Kaaba, the Islamic holy shrine in Mecca, seven times -- she estimates that she will walk 16 miles in a single day.

"I want to be in good shape," she said. "I'm traveling with a group, and I don't want to slow the group down." Mirza plans to walk around the Kaaba from the top tier of Mecca's Grand Mosque. It's less crowded that way, but it involves more walking -- each trip around the top tier is about one mile.

To prepare, her fitness routine included daily 11/2 mile walks outside or on her treadmill and weekly aerobics classes at the Carrollton Senior Center. She drank lots of water and dieted so she'd carry a little less weight. She took yoga to keep herself limber.

"Our prayers require you to kneel and then stand up, over and over again, so it's very difficult if you are stiff," she said.

The hajj culminates at sundown Feb. 1 with Eid al-Adha, the Festival of Sacrifice, which commemorates Abraham's willingness to sacrifice his son in obedience to God's command.

The hajj centers on Mecca -- the city of the prophet Muhammad's birth -- and the Kaaba, which Muslims believe is the shrine built by Abraham when the site was given to him by the angel Gabriel. But the pilgrimage also takes hajjis to three other cities: Mina, Arafat and Muzdalifah.

The Saudi government expects 2.5 million Muslims for hajj this year, with half from abroad -- more than 170,000 from the United States and Europe. (While the number of Muslims, currently about 1 billion worldwide, is growing, Saudi Arabia limits the number of special hajj visas issued each year, because of capacity constraints.)

Hajj is one of the five pillars of Islam. All Muslims in good health and financially able are required to make the journey at least once in their lifetimes.

Many pilgrims are elderly, if only because the cost of the trip may require a lifetime of savings. (Travel packages from the United States cost $2,000 to $5,000 or more.) Borrowing money to make the trip is frowned upon in Muslim tradition.

Keeping pilgrims healthy presents a major logistical challenge for the Saudi government. This year, pilgrims from the Congo won't be allowed into the kingdom, because of concerns about Ebola. Chinese pilgrims will be admitted, despite the severe acute respiratory syndrome scare, but officials have equipment ready to test for the virus if needed.

Twenty hospitals have been readied to meet pilgrims' needs; almost 10,000 doctors, nurses and other medical personnel will be deployed for the event.

Although only one hajj is required, some Muslims make additional trips. Often, this is done for the sake of a parent or other relative who has died or is unable to travel. For Mirza, an administrative assistant at Texas Woman's University, it's simply a matter of wanting to experience it again.

"There's an intense feeling of unity, of the grandeur of faith, of the majesty of the lord," she said. "You feel close to the creator, and it is a beautiful feeling. I wanted to go one more time while I have my health."