It's that first 24-36 hours that's tough for me, after that I am "warmed up" or whatever and do alright and do fine with the pack on, even heavy. Big ups always suck though regardless of how many days in. Nice when they come late int he hike though so you've already consumed a lot of your pack weight.
Day hikes, overnighters or multi day and do you have other people to split up the food?
Feel free to skip all this, it's mostly rambling about food (very near and dear to my beleaguered heart these days). For day hikes and some overnighters I just do GORP (for anyone that's unfamiliar with the term, just google it; there's a million recipes and it takes a while to find the one that works for your body). For full weekends or easier trips I take a mix of GORP, hot chocolate, tea, soup mixes (whichever works for you) and we try to fish. For long trips, anything big calorie and small weight cause I'm now old and fat.
This all assumes easy access to water/snow and fuel/stove.
If you can get a (or several) friends to base camp for you, then you can go all out. Look for friends that like the
idea of backpacking but not necessarily the full monty. What you do is pick trails that are super easy to day hike and bunny hop base camps. Whitney is a perfect intro example of this for your prospective base camp buddies. Base camp friends and you meet at portal the day before and pack them up with full car camping gear, set up and spend the night before you summit. Base camp friends explore the creek trail pond in the morning, have beers at the store and day hike up to the lake and back while you're doing your summit and back. That way, you get back to portal and have steaks/real food for dinner after killing yourself on pancakes that morning/afternoon when you get back. Also everyone is relatively acclimated for what they are doing that way.
After that, the next day you drive an hour over to the base camp area (horseshoe meadows for Mt Langley and do the exact same thing after restocking in Lone Pine and grab another (easy) 14'er and let your base camp friends explore the days worth of beautiful day hikes around there. Only catch is you can't stay more than a night in the same spot at horseshoe these days but it's not a big deal to move.
Couple days of easy, gorgeous base camp day hikes for the lightweights, incredible food with good friends and pretty stunning sierra mountains. I'm sure most mountain ranges with good park services will offer something similar even if they aren't in thin air.
EDIT: Oh, Justin's peanut butter (double duty for GORP mix), honey (double duty for tea) that kind of thing is pretty great and I prefer it to most power bar type things. Not quite as efficient but a hell of a lot more enjoyable. I put them in those goop tubes a lot for day hikes or keep the sealed packs if bear can stuffing. Anything you can get that hits more than one (or better yet two) functions the easier it all becomes.