Sciacchetrail

Maybe you can contact through this site: https://www.parconazionale5terre.it/Esentieri-outdoor.php -- to determine how disruptive it will be to your plans. Maybe not all trails are closed.
Other coastal places to hike in Tuscany include Montemarcello and Portofino parks.

The event schedule indicates the main race starts early Saturday and it seems other activities are peripheral and almost all are in Monterosso. This is a special interest event so it's not going to paralyze everything: https://www.sciacchetrail.com/sciacchetrail-2025-il-programma/?lang=en
The CT trail network is extensive with 48 hiking trails so there are always alternatives. This article in Italian describes the Sciacchetra trail closures and suggested alternatives: https://www.cittadellaspezia.com/2025/03/04/verso-sciacchetrail-fra-chiusure-sentieri-e-alternative-587313/ The Tourist Info Points staff can give you alternatives based on your skill level.
The bigger concern in late March is the weather. The ferry is not going to be running yet, and a rainy period would be inhibiting. One under appreciated attraction is nearby La Spezia, a pleasant city with some interesting museums - art (Lia), Naval, and ethnographic; a nice waterfront; and a castle with an archeological museum. https://www.visitspezia.it/en/art-and-culture/museums-and-library.html

Hi Mike - regarding the alternative routes....
We didn't get to do this one but I've read many times that that the coastal route from Levanto to Monterosso is a nice hike. I believe that's 591 (ex 1) Levanto - S. Antonio Mesco +590 (SVA 10) S.Antonio Mesco - Monterosso, and is one of the recommended alternates. Given dry weather, I'd jump a train to Levanto and hike south to Monterosso.
Did you get to do this one per chance so could provide a firsthand account?
Details about this route:
591 (former 1) Levanto - S. Antonio Mesco
https://www.parconazionale5terre.it/Eiti_dettaglio.php?id_iti=3573

590 (SVA 10) Monterosso - S.Antonio Mesco (OP would be doing this backwards from S.Antonio Mesco):
https://www.parconazionale5terre.it/Eiti_dettaglio.php?id_iti=3572

landersen24, I'm going to assume you have reserved your accommodations? Where in the CT will you be staying? I will agree that the bigger concern in late March could be the weather but at least there's more than just one day to work with so if you have a place to park your boots, I don't know as I'd cancel.

Thank you all for the information.
We do have a hotel already reserved in Corniglia.

I’ve been watching the weather on date and time.com and also Meteoblue.com and it is showing a ton of rain right now - scary because when we researched this time of year it showed little to no rain. Does anyone have more experience with this time of year and would it be best to cancel?

The problem is that no year/month/week is exactly the same. Some early spring visitors may have experienced a mostly dry couple of days, and others were handed mostly wet. But here's something to consider? Sciacchetrail appears to be a big deal. The CT trails can be slippery and dangerous when wet, and often close entirely for rain. It's possible the organizers chose the time of year they did for it - other than avoiding the months of heaviest crowds - because there's a reasonable chance of decent weather. Like anywhere else, there's no guarantee but something to think about? Again, you also have more than one day to work with.