I don't think you need to worry that much - my battery will easily make it a full day if I keep the usage light. It is possible to kill the battery in 4-5 hours, but this usually involves being in a low/no signal area, running poorly-written widgets that update too often, or playing high-end video games constantly. Even calls, texts, and a little web surfing shouldn't be that bad.
The one real unknown is the 4G LTE radio, though. Nobody's used a LTE radio in a phone yet AFAIK, so we don't know how it will affect the battery life. It could be a battery hog from being relatively untested baseband chips, or it might actually be better because the 700MHz signal gives better signal strength or the higher data rates mean less time transmitting or something. In any case, there will probably be an option to turn off the LTE radio if it turns out to be a battery hog and you need the extra life more than the extra internet speed.
If you're really hard-up for battery life, you can turn also off mobile data entirely so the phone only gets phone calls and texts, and that should be about as good as it gets. I've never actually went a long time with mobile data off, but I wouldn't be surprised if you could get 2 days out of it like that.