I don't know the exact circumstances of how you came to accept an offer without knowing what you'd actually be doing, which sounds to me like accepting without other crucial information like e.g. what salary is or whether you'd be HQed in Mountain View or Timbucktu, but you can quit a job any time you want, for any reason, and the same goes for offers. I'd be appropriately apologetic with your point of contact at the company, but "Thanks for getting us this far. The actual position which the company proposes placing me in is not what I envisioned during our earlier discussion. I'd still like to work for you guys if you can be flexible on that, for example by offering me $BETTER_POSITION." If they're not flexible, then say "OK, I appreciate where you're coming from. You only want to hire the people who'd do their best work as $OFFERED_POSITION, and I only want to work in a position which is the best fit for my skills, so I don't think having me do that job is a mutual win for us. Accordingly, I think we shouldn't move forward on this."

They'll might hem and haw a bit, but ultimately, you have an absolute right to not work in jobs you don't want. You'll probably burn your bridges with that decisionmaker, and more than likely with that company, but the world has many companies.

For your future reference, and that of HNers in the peanut gallery: nail down major specifics like this before saying Yes. If you hadn't said Yes already, you'd have an absolutely not-in-the-least-bit awkward opportunity to continue the negotiation and say "That position doesn't sound like what I'd like to be doing. You should offer me a better one, or alternatively, compromise on one of the other levers in this negotiation.", and if that negotiation broke down it would cost you nothing and not leave anyone with a bad taste in their mouths.