nod along while she talks, try to steer the conversation to other topics and pace yourself so you're done eating at the same time as she is.

or, 'oh, i know - but i had such a big breakfast/lunch earlier, that i honestly can't fit another bite. this just looked too delicious to pass.'

alternatively, 'oh, sorry, not eating here today, meeting friends for (meal).'

the idea isn't to try to fight a losing verbal battle with her, it's just to get out of an awkward social conversation and keep doing your thing.