I did R2R in 2015 and I have summitted Mt Whitney twice (2013, 2016). I found R2R orders of magnitude harder than Mt Whitney. The trails are setup more for mules than humans, and it is hard to get an even gait while hiking so I found I expended more energy. The heat affected me more than does the lack of oxygen affects me on Whitney. Of course, this was mid-June and AZ was in an early heat wave, it was around 111 that day.
