This is a great opportunity to improvise. You could mix up some of your repertoire with improvisations and get that 1 hour worth of music. I wouldn't be too worried about playing single movements from sonatas. The situation sounds more informal and there is no need to impose concert etiquette.
I didn't even know that was part of concert etiquette just that both of the movements I know end with "attacca" so it sounds kind of funny if I just stop... I don't know...
I agree with Bob, people are not really looking for 1 hour of virtuosic music. Just have them enjoy the moment.
But I have another question. My sister told me I should avoid playing the Chopin Nocturne (in b flat minor). She said it was too sad... ?