grammar - "Would prefer to do something " vs. "would prefer doing something"

I've heard that it is grammatically incorrect to use a gerund after would prefer, for example, you say "I'd prefer to walk" not "I'd prefer walking".
But I'm almost sure that I have heard a sentence like "I'd prefer walking to driving".
I was wondering if it is because in the former there is only one option, whereas in the latter there are two options and you are comparing two things.
I know we can use a gerund after prefer, as in "I prefer driving." Is it also possible to use it after would prefer, and say "I'd prefer driving"?

Answer

You can use gerunds if you're using would in a conditional statement, e.g.

I would prefer walking over driving if my legs were in better shape.

But if you're just making a general statement about your preferences, would prefer is normally followed by a verb.