When we were in France in July 2022, I rented bikes for my family to cycle from Blois to Chambord. They picked up bikes at Detours de Loire, as it was right near the train station so it was convenient for our group coming in from the train. I didn't end up going on the trip as my daughter got sick that day, but overall I think their experience with the bicycles was pleasant. It's about 13-14 miles each way. They followed the paths/roads along the Cosson river to Chambord, then went north and followed the Loire river back down to Blois. There's also a bike path south out of Blois part of the way towards Chaumont, then you are on the road the rest of the way. That said, do note that the bike paths in that area seem fairly broken up - so most routes are a mix of bike path and then roads.
https://www.detoursdeloire.com/
https://www.detoursdeloire.com/la-loire-a-velo/