I rode that route twice; once with local friends (see pictures), anoher occasion with friends from other countries.
Day 1: We started Nikko in the morning, rode to the top of Hosoo Pass (see picture), to Kiryu where we stayed at a small ryokan (see picture with proprietor) near Kiryu station.
Day 2: We started from the ryokan in Kiryu, rode to Watarase basin, where there is an observation deck(see pic) to Sekiyado Castle, rode along the Edo River all the way to Tokyo.
Some pictures from that tour uploaded. The places are self-explanatory by the file name.
The female rider (see picture) of age over 60 rode/climbed the Hosoo Pass without stopping, without waking, to the summit. That suggests that if you are an "average rider," there should be no problem in doing it in two days.
I hope this helps,