I'm just thinking aloud on a way to define the subgenre - most stories considered epic qualify, but there are more modern ones that are described as "epic" by their publishers, but in structure may be much closer to YA fiction.
I think the key is that writing is not singular. You get a few, and they are typically a few, books/series which fit a sub-genre perfectly. Sometimes they are even the ones that start a specific sub-genre off or promote it into more active use.
The thing is most books are not singular; they straddle multiple sub-genres at the same time. Heck some even straddle genres themselves (Pern would, in my view, straddle both Sci-fi and Fantasy - since its got a sci-fi base but many of the stories take place in what most would consider a very fantasy setting).
Epic to heroic as I would define them is:
Heroic - focuses upon a singular hero or very small group of heroes. Typically told from a single viewpoint or narration primarily following the single hero/group. It would be a good book length, but nothing too extreme and a really classic example might have each adventure fitting into a single book (rather than a long series - though there might be overarching story lines).
DnD type adventures stories would oft class as heroic - but then again many would also class them as very DnD like.
Epic- typically multiple points of view and might not even have a single "leading" character. It would also expect to be long in terms of pages and expected, but not required, to use more extensive description and scene setting
I think its easy to get too bogged down in the specifics and a lot of the time many personal opinions can sometimes (overtly or subconsciously) be a case of having a sub-genre a person likes and thus trying to promote as many books into that sub-genre as they can (whilst sometimes at the same time excluding those they dislike).
Publishers have their own ideas, but its my view they will use the terms in a very loose manner - typically whatever is "Hot" selling will be promoted through their choices, but also get the label slapped on it too. As we see today with GRRM promoting EPIC fantasy we will see more epic tales and some that we could argue are not quite "epic" getting the title from the publishers.
I'd also argue that most people only pay a light interest in sub-genres. Mostly they use it just to find similar works; its a filter - a simple way to casually filter the vast number of published books into something more manageable and similar. I say rough because one epic tale to another can be vastly different.
