I was staying in Blois and rented a bike from this place to Chaumont-sur-Loire 20km away. I had a…read morefantastic ride! The Loire à Velo paths are super easy to follow and the guys here will give you a map of the routes.
It was 15€ to rent a bike for the day, plus a 300€ deposit (which wasn't really a deposit; the guy just wrote down my credit card number and said he'd only charge me if the bike was lost or damaged). The basic bike is made for all-terrain, which comes in handy for those charming French cobblestone streets as well as paved bike paths. It took a bit of practice to shift gears, but they worked well, as did the brakes. The seatpost doesn't require a special tool to adjust so you can move it up and down as you need to. They also give you a repair kit, small pump, cable lock and key. I didn't get a helmet, nor do I feel like I needed one. The bike paths are so smooth and easy to ride, and I was passed by maybe three cars all day.
It's also easy to take your bike on the TER (regional) train in one or both directions for your ride. One of the train cars has big drawings of bikes on the side, and just when you enter, you'll see 3 hooks where you hang your bike vertically by the rim of the front tire. It was very easy to do.
You can also arrange to drop off your bike at one of the many partner shops throughout the Loire, so for example you could get your bike in Blois, ride to Amboise, then leave the bike in Amboise and take the train back.
I definitely recommend renting a bike as an unforgettable way to see the Loire and its chateaux!