TCC has covered that route a couple of times. Route 76 in particular is legendary. Please see the attachment in this thread. I "hikled" it the first time by myself from the south when I lost Christoph and the second time from the north with TCC: the infamous Hadano Loop. It's great fun, but make sure to wear SPD, not SPD-SL cleats, or better, bring a MTB.
Alan, Thomas rode from Tanzawa ko R76 up to R413 a few years back and commented that the road was in terrible condition in places. I think if you're prepared to carry your bike a bit it's passable. Wow, that's the first time that's happened, speak of the devil, he posted at exactly the same time as me.....Nice pic Thomas