Hi
You mention, in other posts, a "centre based" holiday, by that, I assume you mean, staying in Hotel, B&B, or, self-catering accommodation?
We typically do something similar (although, our accommodation is a Motorhome).
We have enjoyed, what I call gentle cycling, in UK, and, mainland Europe, usually, but not always by seeking out Canals, or, coastal areas. Rivers also often provide an "easy" cycling valley to follow.
So, for example, in UK, we have cycled around:
Staveley area, Derbyshire, giving access to Lancaster Canal
Newcastle-upon-Tyne, Hexham, etc, giving access to Hadrians Way Cycle Route, and, if you are up for a few hills, the Northumberland Coastal Coast and Castles route.
Peterborough, well served with dedicated cycle routes
York, well served with dedicated cycle routes
Brighton / Littlehampton area
Hanley Swan, Worcester, rather more hilly than we usually tackle, but, we managed to enjoy it!
Denham, Buckinghamshire, access to the Grand Union Canal.
I am am unsure of your age profile, but, we are 72 and 73 years old, and, typically 20-30 miles per day cyclists