Würzburg is very nearly in the middle of the five-hundred-kilometer Main Valley Bicycle Route (Maintal Radweg). From here you can cycle 246 km upstream to Bayreuth via Ochsenfurt (top sign in the photo) or 243 km downstream to Mainz by way of Zellingen (bottom sign). The middle sign points to the main railroad station in Würzburg (Hauptbahnhof), in case you have finished cycling and want to take your bicycle home on the train.
I have cycled most of the lower half (Würzburg to Mainz) of the Main Valley Bicycle Route at one time or another, some sections more than once, but with each passing year it becomes less and less likely that I will ever do the upper half.
The General German Bicycle Club ADFC, of which I am proud to be a member, has a system of testing and certifying long-distance bicycle routes (longer than 100 km) and rating them on a scale of one to five stars, based on criteria such as being car-free, being clearly signposted, being paved or otherwise usable in all kinds of weather, going through scenic and/or interesting regions, being reachable by train and having an ample supply of certified Bike-and-Bed accommodations along the way. The first route to be awarded all five stars by the ADFC was the Main Valley Bicycle Route, which I can also highly recommend from personal experience.
By the way, the name of the river Main rhymes with Rhine and is pronounced like the English word mine. It has nothing to do with the English word main.
This is the guidebook that I have been using for cycling in the Main River Valley. The one I have is the sixth edition, updated and published in 2002 by the Austrian publishing company Esterbauer.
They are currently selling their twenty-first edition, published in July 2021. Apparently lots of cyclists still like to have a printed guidebook, with detailed maps, even if they are also using a GPS application.
My photos in this post are from 2005. I revised the text in 2022.