You probably have to go now that it's arranged, or amend it to a cafe if you can.
Don't be harsh with yourself ( or your acquaintance), but be prepared to refuse requests/demands for future meals or things she might want you to go and help with.
Maybe you can slip into the conversation before she can ask for help/more get-togethers that you're finding it very hard to fit in everything you want to do and meet up with long-term friends/family and have realised you have to cut back on some things.
Compassionate, kind, but firm is possibly what's needed. Might be worth researching info around where she lives to have some suggestions for other activities she can get involved in if she's lonely.
I can get roped into things then find it's hard to pull back! Be realistic about what you could or want to offer in the way of time/friendship and think about your own wellbeing too.