The Rick Steves Germany guidebook describes this 20 mile ride as "strenuous" in some parts. I'm concerned about my wife. She can handle 20 miles, but I'm concerned about the uphill sections. I don't want her blowing a heart valve! How strenuous is it?
I would suggest renting electric bikes if you have any concerns. Almost every bike rental shop now offers ebikes at a good rate. We did this to ride around Tegernsee and everyone in my group agreed ebikes made the experience that much more enjoyable.
DJ
If I recall it correctly, there are two steep but short segements only (the sum of all ascents and descents is ca. 300m); the rest is modestly hilly or even flat. Here you can find some pictures that give an impression: https://www.allgaeueralpen.com/2014/08/stausee-radtour-einmal-um-forggensee-gegen-uhrzeigersinn/. Pick up a boat schedule at the Füssen TI so that you can put your wife into a boat at one of the eight stops if she cannot go on any more.
The Füssen outdoor app has offline maps.
That area is hilly, but his you have a 30 gear road bike, it is not mountainous.
I am an avid cyclist and have done 100 mile bike rides in one day as well as 500 mile rides in a week. I am 70 and still do 30 miles at a time easily. Twenty miles is not much at all. The worst case would be you had to stop and walk your bikes up a steep hill.
I suggest going on the internet, pull up a map of the route, try to determine the altitude as well as the grade of the route.
https://www.tripadvisor.com/ShowUserReviews-g198564-d1731437-r118869918-Forggensee-Schwangau_Upper_Bavaria_Bavaria.html
Thinking of the same for next year, with the same wifey problem. It's classed as a 'light' ride, and you can rent electric bikes if your euro extends that far. There are many more suggestions on the Fussen website:
Thank you all for the suggestions.