It's not just conditioning, it's pacing. Accepting the heavy load and a slow stride will get you pretty much anywhere. I think it took me 8 hours to get to trail crest with that big load.
I remember heading up the switchers that day, stopping every 10 switchbacks to sit and rest for at least 5 minutes. Only casualty was my pants-seam, which split wide open, I'm embarrassed to admit. I spent the next 2 weeks running around the Sierra with duct tape holding my pants together.
No hurries, no worries.
