09/03/2013 at 07:20 PM EDT
Last January, actress Valerie Harper sat in her doctor's office, listening as he broke the grim news that she had terminal brain cancer
and might not live through the spring.
But for the past two weeks, the actress has been enduring grueling four-hour-a-day dance rehearsals in anticipation of her Sept. 16 debut on Dancing with The Stars
"I feel much better," Harper, 74, tells PEOPLE. "And my brain scans are looking better too. The doctors tell me there's less evidence of cancer
, which is very unusual. However, they both say it's not a case of if, but when. And I can live with that."
Harper broke the news that she has leptomeningeal carcinomatosis, a rare, lethal form of cancer that strikes the membranes surrounding the brain, in PEOPLE this past March. Her inspiring attitude of living each day to the fullest touched millions, so it's hardly surprising that the former Broadway dancer signed on to appear on the hit ABC show, dancing beside Tristan MacManus.
"When they asked, I thought, 'Why would I say no?' " says Harper. "When life asks you to dance, you just have to dance."
Her biggest fear, she insists, is messing up. "I'd been asked to do this before, but I always said no because I started dancing on Broadway at 17," she says. "It helped pay for my acting lessons. And I didn't want to appear on the show and fumble because of my diagnosis."
But ever since June, her doctors have been "ecstatic" with the results of her brain scans, which have begun showing increasingly "less evidence of cancer." Buoyed by the diagnosis and physically feeling better than she has in months, Harper hopes it "might prove inspiring to people to see a 74-year-old woman with terminal cancer dancing."
Harper is so serious about her performance on the show that, with her doctors' blessings, she's rearranging her treatment schedule for the powerful medication she's been taking to combat her cancer.
"After she takes it, she doesn't feel good for about a day and a half," says husband Tony Cacciotti. "So right after the show on Monday nights, she'll take it and then take a break from practices until Wednesday afternoons."
The veteran hoofer, who has been spending recent evenings soaking in Epsom salts after days of polishing her foxtrot and cha cha, vows "to do my best" on the show. Now, if she could just find a way to fit into those costumes.
"The other day," she laughs, "I told the wardrobe supervisor, 'Your job is to take six inches off my butt.' "