Yea, the Devils Path is a pretty good hike to train your legs. We did half of it last year. Just make sure you get used to being on your feet for 12+hrs with a pack at altitude for a Whitney-type climb.
Years ago, Half Dome from Happy Isles was my first real hike/climb as an adult and honestly, coming from sea level (like you), it was very hard, especially with the relative elevation and we did it as a dayhike. I learned some things from that hike and came back a couple years later and did it again more prepared and felt much better, enjoyed the climb and shaved an hour off my ascent while feeling less tired.
Don't be discouraged with Whitney. I think everyone is just making sure you know what you're getting into. As Whitney Main trail is such a popular route, many hikers come from sea level unprepared (not that anyone is suggesting you would) and there have been people injured/rescued/killed or just hitting the wall and need help getting down. We want to make sure that (1) you enjoy your trip and (2) ascend and descend safely.
One question - have you ever been to higher altitude or hiked anything higher than Half Dome?
