Will be doing Gorgonio this weekend and taking the Vivian trailhead.
I did San Bernadino last weekend, 17 miles in 7 hrs. I know Gorgonio is very steep and hard so Im thinking it will take another couple of hours.
The first time I hiked Whitney was in 2011. Our last big training hike was San Berdoo and we did it in 9 hrs. It sounds like you're in good enough shape for Whitney, just a question of how to adjust to the altitude.
If you're looking for another training hike, here's a good one:
Mt Baldy, via Bear CanyonDistance: 12.8 miles RT || Duration: ~6-8 hrs || Elevation Change: 5800' || Max Elevation: 10,064' || Difficulty: Very Strenuous
As far as hiking alone, you may look into renting a SPOT:
http://www.satellitephonesolutions.com/rental/sps-rental-program/sps-spot-rental.htmAs noted in other threads, they do have their limitations, but there are members of this board that use them whenever they hike solo.