If you're creating iterations for a single project then it's quite easy to import them using the Excel Add-In for Agile Central. If you have a lot of projects on the same cadence then you're better off creating iterations through the UI but you can create them in a parent project and propagate them automatically into the child projects. Either way you're doing the work once per iteration (at most) and it only takes a few seconds so I don't find it all that tedious, and I manage all of the timeboxes for a very deep project tree. Hope that helps.