Bears never go above Whitney Portal, so that's one worry you can forget. Bears don't attack people at Whitney Portal either -- they only sneak up and steal their food. If you are starting your hike in the dark (which you shouldn't be since you are backpacking) you will be just fine on the bear issue.
With your shorter hikes on a regular basis, you should be ok with the long day. Your legs will be really tired, but you will be surprised what the body can do.
