Congratulations Couch or your successful summit. My wife and I have tried to summit as a day hike each of the last three years (not counting this year). and failed each time. We did get farther each year and last year we were probably less than a half mile from summit when I just completely gave out of energy and knew we had a long way back. I was so glad to get back to the trailhead. We are both 65 and live at sea level in North Carolina so don't think another attempt would be any better. Each year was a good adventure and the hike was beautiful until exhaustion set in. We enjoyed meeting other hikers on the trail and one shared an energy bar that was better than anything I had and probably eating it is the only reason I got back to trailhead before dark.
Thanks! Don't give up yet - spending more days at altitude before the hike might do the trick, and if that doesn't work, you can always try a backpacking adventure from Cottonwood Lakes or Onion Valley to Whitney. I also met a guy in his 60s who went up from the Portal to the summit and back twice within a 4-day period. That was pretty impressive!
Interestingly enough, I had no appetite on the way down. Even after arriving in downtown Lone Pine, I finished less than half of the tasty, juicy 12 oz ribeye I ordered that evening. My appetite didn't return to normal until the following day.