You've got a plan, you've budgeted your time, you have your supplies ready ... and it still seems like way too much work for one person. There's really no shame in asking for help. And you might even need it for some of the big projects that involve moving around furniture or climbing ladders to dust light fixtures. Getting someone to help you clean might be even harder than getting someone to help you move, though.
If any other people live in your house, that's a good place to start. Their stuff and their mess are contributing to the need to spring clean, right? If you have kids, get them to focus on their rooms first (including the closet). Just remember that you'll need to help keep them on track and make sure they didn't just shove everything under the bed. Even small children and toddlers can do things like dust baseboards; they're still eager to help, and there's no time like the present. No luck with anybody at home? Offer to help a friend with his or her spring cleaning in exchange for help with yours (or for free food and/or alcohol -- if it works for moving, why not for cleaning?). Finally, if you can afford it, you could also hire a maid service to tackle some of the larger jobs that just seem way too overwhelming.