Lake Biwa is the largest freshwater lake in Japan. It also supplies most of the water for the Kansai region, and if they can use the word "fresh" to describe the stuff coming out of my tap then something is definitely wrong somewhere.
Apart from one hilly section up north, the road is flat and can easily be cycled in two days - and at least one cycling marathon has participants do it in a single day. Cycle-tourists don't need to push that hard; explore some of the towns along the way, see the castle at Hikone and the windmills nearby, and by all means don't forget your bathing suit if you're traveling during Japan's hot, muggy summer.
Note that unless you want to bypass the shore on a rather busy highway (route 303), you will have to do some climbing (to almost 400m) at the peninsula at the extreme north end of the lake. For your trouble, you'll be rewarded with some of the best views on the entire route. Elsewhere, this part of Shiga Prefecture has some communities that look as if they haven't changed for half a century or more.
My advice is to get a detailed map and take small roads as close to the lake shore as possible. There are also short bikepaths here and there, though they go off on tangents and generally won’t help you get around the lake.
