Actress Francesca Faridany has never been through the kind of marathon she’s running as Nina Leeds in Eugene O’Neill’s “Strange Interlude.” As the complex heroine at the center of what was originally a nine-act play, Faridany is onstage for practically all of the Shakespeare Theatre Company’s nearly four-hour show.

“Sleep,” the British-raised Faridany says in her Queen’s English accent, explaining how she gets through five-show weekends. “And gallons of water.”

At Arena Stage, Helen Carey is charting her own steep course. Carey is playing Mary Tyrone, the morphine-addicted woman at the core of the family agony in O’Neill’s “Long Day’s Journey Into Night,” clocking in at just under three hours.

But the agreeable Carey says she’s been through worse. Years ago at the STC, Carey was in Michael Kahn’s four-hour, high energy edit of Shakespeare’s three “Henry VI” plays, rehearsing the lead in “Antony and Cleopatra” at the same time. Between the matinee and evening shows of “Henry VI” the cast would dine on meals brought in by the theater, then nap.

“You’d eat in the costume you finished in, lie down, get up and start all over again,” Carey says.

An epic epidemic has been running through Washington lately, with a flurry of shows flirting with three-hour running times and beyond. The O’Neill festival at Arena and the STC is largely responsible; Arena’s just-closed “Ah, Wilderness!” strolled in at 2 hours 45 minutes, and so does “Long Day’s Journey,” which took the usual four hours to perform at Arena in the 1990s. That production’s Mary was Tana Hicken, currently playing Faridany’s mother-in-law in “Strange Interlude” — a play that lasted five hours in its 1928 premiere, but that director Kahn has compressed to a comparatively brisk 3 hours 45 minutes.

Even the tourist-friendly musical “1776” at Ford’s Theatre demands stamina, with the singing, dancing and debating carrying on for 2 hours 45 minutes. At least the Ford’s actors get Sundays off; they play two performances on Fridays and Saturdays. At Arena and the STC, it’s a five-show weekend — Friday night, and two shows on Saturday and Sunday.

Faridany and Carey have the largest and trickiest roles in their plays, so the heavy schedule means taking particular care to maintain body and soul. Not surprisingly, given how much time they are spending in darkened theaters, both women chose to sit in the sunshine during interviews at coffee shops — Carey near her longtime home in Falls Church, and the New York-based Faridany by the STC-provided apartment close to the troupe’s Lansburgh Theater. (The show is in the STC’s larger Harman Hall.)

Also not surprisingly, both performers have been building something new into their routine: a professional massage (a luxury that Faridany pronounces with the emphasis on the first syllable).

Carey told her masseuse, “You’ve got to get my shoulders down from around my ears,” adding of the long, jittery role in O’Neill’s most personal drama, “It’s tense-making.”

Through the first five-show weekend, Faridany hadn’t figured out exactly how to eat. A full meal would have been too much before the matinee, yet she needed sustenance to get through the show. So backstage, she tried apples.

“Disaster,” Faridany says. “That just gives you a sugar rush that plummets about 20 minutes later.”

Carey turned down several opportunities to play Mary before. (When asked her age, Carey says, “Old enough to play the part.”)

But she pursued the role when she saw that Robin Phillips, a major Canadian director she had long wanted to work with, was going to direct at Arena (and she’ll give it another whirl next year at the Guthrie in Minneapolis, in a new production with current co-star Peter Michael Goetz).

The heft of the role lies in Mary’s battle with addiction and the way her mind flits across the years. “If your concentration goes, God help you,” Carey says. After the first long weekend, Carey felt the wear of all the talking on her vocal cords. Her voice is naturally deep to start with, and she laughs, “I think I’ll probably be a bass by the time we finish the run.”

For Faridany, the rare opportunity to play Nina Leeds came at a time when she wasn’t looking to stray from New York. She and her husband, director and translator Stephen Wadsworth, have a daughter who’s nearly 2; the toddler is here with Faridany and their nanny while Wadsworth directs “Don Giovanni” at Julliard, where he teaches. Yet like Carey, the actress pursued the role, intrigued after participating in a reading Kahn held in New York.

The notorious challenge of “Interlude” is mastering not only the dialogue but also the rampant subtext that O’Neill puts into words. A few years ago, that might have been debilitating for the 42-year-old Faridany, who worked herself into a case of stage fright playing Rosalind in “As You Like It” for the STC. Parenthood, she says, has helped simplify her focus. So has advice a doctor gave her mother when she was dying of cancer several years ago, something about “energy dollars” and spending them wisely.

So now Faridany talks about “staying in the moment,” relaxing her jaw when it begins to play havoc with the tension in her neck and tongue, not “freaking out.”

“You just say the next line,” she says — though during the last performance of her first five-show weekend, fatigue kicked in and a word she could think of simply wouldn’t come out of her mouth. By Sunday, that happens to all of the cast, she says — to the “Long Day’s Journey” troupe, too, Carey testifies — though nearly always in ways too fleeting for audiences to catch.

“Your brain is tired, your muscles are tired — everything’s tired,” Faridany says, even though she, like Carey, exudes a high-spirited energy. (“Life is good,” she smiles.)

Carey, whose daughters are grown and whose husband is retired from the foreign service, suggests that getting through long weekends in big roles is “like a sport, where you spend a hell of a lot of energy trying to get the form right, and get the timing right. Once you get it, you can let go of all the extraneous effort.”

Still, naps at the theater are par for the course; Carey has what’s called an “Equity cot,” mandated by the actors’ union, in her dressing room. And Faridany is trying to strike a balance that won’t have her daughter complaining in 20 years, “You were asleep all the time!”

Carey seems to have the better of it, declaring simply, “I have decided that on Monday mornings, it’s me and the masseuse.”

Strange Interlude

by Eugene O’Neill. Directed by Michael Kahn. L. About 3 hours 45 minutes. Through April 29 at Sidney Harman Hall, 610 F St. NW. 202-547-1122.

Long Day’s Journey Into Night

by Eugene O’Neill, directed by Robin Phillips. About 2 hours 50 minutes. Through May 6 at Arena Stage, 1101 Sixth St. SW. 202-488-3300.