Just as I suspected, Marilu is a contender. I think she will last to the finals.
If you look at Marilu's Broadway credits:
https://www.ibdb.com/broadway-cast-staff/marilu-henner-71729
She started out as an actress/dancer/singer on Broadway, in
Grease, as a replacement, then the following year went on to originate a role on Broadway and then did a third Broadway musical. She is Broadway trained. Then she did
Taxi. She's not one of those stars who comes to Broadway after becoming famous in Hollywood, and they overlook she can't really dance & sing very well, because she's able to pull in the box office numbers with her fame. She knows the rigors of being 8 performances a week and legitimately being cast because she earned her castings. She also kept coming back to Broadway musicals after being a long-time regular in other TV series, so Broadway is her roots & passion. And it seems she uses the fame from those TV series to come back to Broadway.
I think she continues to train as a dancer even when she's not in something, and maybe does yoga and clean eating, even before that was a trend. It's part of her work ethic. She never knows when she's going to get a call to interview to be in another Broadway show. I vaguely remember her being interviewed,

possibly on The View. It might have been when she was on Broadway in
Chicago, and she mentions how much she still dances, does yoga(?), eats well, and all she does to stay in shape for her age.
Oh! Yes, here it is, she made a "Dancerobics" video after being in
Chicago.
https://www.amazon.com/Marilu-Henners-Dancerobics/dp/6302492068/ref=cm_cr_arp_d_product_top?ie=UTF8
And I could have sworn that one of the women on The View had asked her if she was interested in being on DWTS and she said she was. They might have even asked her, but she was too busy at the time. (Obviously MY memory isn't as good as Marilu's.

)
In 2015, she started doing a Hallmark TV movie series with Candace Cameron Bure, as her mother. It was less than a year after Candace was on DWTS - who subsequently was cast as the star of the Hallmark TV movie series and her stint as a co-host on The View. Candace & Marilu have made 4 movies together since. I would think at some point, Candace would have said to her how rigorous the training was during DWTS, if Marilu didn't already know.
DWTS might have asked Marilu to be on the show after the first movie, as Marilu could have picked up some of Candace's fan base, since she's playing her mother, (along with her own
Taxi &
Chicago fans.)

And she might have even put them off a season so that she could train to make sure she has the stamina to last the whole season. Donny Osmond did it. He openly campaigned to be on the show and worked out & trained to make sure he'd last through the tough hours. He comes from that same generation & work ethic. And he won the mirror ball.
I think, if she still wants to come back to Broadway, the way Chita Rivera does, (Chita's last show was last year at age
82,) the last thing Marilu wants is for Broadway producers to think that she's too old & tired to be in a Broadway show. Especially with her memory, she'd remember people talking like that about her.

So, I think she has been preparing.