My 4-day trip started with a night in the Onion Valley parking lot. Hiked all day, about 15 miles, overnight at Center Basin junction, just north of Forrester Pass. (Also had my great <a href= "http://www.mt-whitney.info/viewtopic.php?t=92" target=_new> Wild Bear Encounter </a> there.) Second night at Wallace Creek (another long 15-mile day) Third day got to Guitar Lake and had a mid-day dinner and rest for about 2 hours, then climbed to the peak, spent the night on top. Fourth day, Portal by 2:30, back home to Fresno by 9 pm. The first two days were very long and tiring.
Yes, it is reasonable to go from Guitar Lake over Whitney/Russell pass to Iceberg. I did that with a full pack (in reverse) years ago.
For conditioning reference, I'm over 50, but run 10 miles per week (around 7:30), and ride a bicycle to work and back several times per week.