Ken 2, if you were to extend that loop to, say 100 miles, what would you add?
Turn around and do it again the in other direction! :)
Instead of exiting over Bishop Pass, you could continue south on the JMT over Mather, Pinchot, and Glen Passes to Charlotte Lake and exit over Kearsarge Pass to Onion Valley. That should get you to right around 100 miles. The shuttle cost would be higher, of course.
You could also start from a trail head farther north, but I think it's better hiking to the south.
Since you've already done the JMT, instead of extending the end-to-end distance you might want to think about using the extra time for off-trail side trips. There are many great areas to explore in that region. Humphreys Basin, Darwin Canyon, Goddard Basin, Ionian Basin, Palisade Basin to name a few.