Lucy keeps the decoy coffin wrapped up and invisible as she rushes towards her destination.
Avoiding making enough noise to be perceived clearly no matter how invisible she is takes care, and slows her down enough to make her clench her teeth and pointedly avoid calculating how much more ground she would be covering if she didn't have to be discreet.
Despite what Donni said, it takes more than a day. It took their group of six about a week to get to Haven the first time. Lucy in natural form is faster than a Companion, and they weren't exactly bolting at breakneck pace the whole way, but Lucy is still significantly less than halfway there when the sun drops behind the horizon and the traffic on the road thins to nothingness and she can abandon near-silence in favor of just avoiding sounding like a gatling gun made of cannons. She makes much better time this way.
By the time the sun rises again, she hasn't made it there, but she has made it off the main road. She compares her current progress with their schedule on the way to Haven, and slows enough to be quiet again; it's much less likely that she'll be heard without all the constant traffic of the main road, but even with her enhanced hearing there's no guarantee she'd realize there was a witness before they heard her, and she can't make the swap until nightfall anyway.
The acrid smell hits her, and she wrinkles her nose. She doesn't have trauma about it, and she can tolerate smells that would make a hobo puke, but now that she's met Tylendel and doesn't have Vanyel to worry about...
This is a sad place.
Well. She's going to fix it. She's made a good start, and she is at this very moment working to fix something adjacent, and she's going to fix all of it in time.
She makes her way to the keep, and the graveyard, by a circuitous route that keeps her out of the way of people. She's totally justified in what she's doing here but it's still a little awkward.
She lurks there among the dead until the sun goes down, until the sounds of human activity she can detect are few and far away.
She pries into the earth, carefully, oh, so, carefully. She probes beneath the soil until she finds the telltale wooden shape six feet down. She wraps her tentacles around it, engulfing it completely so that even if the coffin disintegrated no part of the body would escape her grasp, and slowly tugs it out of the earth. When it reaches the surface and the ground is pried open as far as the coffin will push it, she holds it open as she finishes extracting the real casket, and pushes the empty decoy into the place beneath the earth where it lay.
Once the transfer is complete, she examines the site as best she can in the darkness, poking the slight discrepancies back into place.
She waits there until dawn, uses the improved lighting to check to make sure she didn't miss anything, and then takes off again as swiftly as she quietly can.
It is shortly after noon of the next day when she reaches the palace again.