Such a hike might be Horseshoe Meadow to Cottonwood Pass, if you have problems getting over it, you will have similar problems somewhere along the MMWT.
Heather, being a fellow lowlander I strongly recommend this route as a prep & acclimation hike. It's very close to the Whitney Portal and is only about 9 miles RT - half a day at the worst. It'll get you well above 11,000 feet and has a series of about 35 switchbacks that take you to the Pass (good practice for boredom control if you wind up hiking Whitney's 97 switchbacks instead of going up the chute).
Oh yeah - and the view from the Pass is gorgeous!
