The female Mallard does not have such a big black cap.
The Indian Runner drake will have such bright yellow bill.
The Mallard drake has an iridescent dark green to dark blue head.
I think this is a healthy multiple-generation hybrid female knowing
that mallards are fornicators and often serial gang bang rapists.
NS, I’ve noticed on my continental travels that there are a lot of Mallard ducks and drakes where the plumage is not the usual. The green head plumage of drakes in particular is very variable!
I have watched a drake rape and the poor old duck was in a bit of a state after six or seven drakes had finished with her!
I would also think the Indian Runner has a different body shape, not as plump and leaner stretched out form.