I think, without a doubt, it's the marathon. Outside of Paula, she's pretty close to the rest of the world. There's a total of 5 active women on the all-time list ahead of her and chances are good that no more than 2-3 of them will be in a given WC/OG marathon with her, if she goes for it. Only Paula's time is truly out of her reach.
She has no chance to medal, ever, in a track event. If she is in good form for Athens, and what I said above holds, she'd be rated at least a bronze contender, maybe better. In the '05 WC she'd even have a better chance, because some big guns will skip that.