Doesn't seem like it will be that long, and yes, the horseback riders are from the same scene. It seems like Bruce's dream is of a dystopian future in which Superman has eradicated much of humanity and rules all that is left. Batman is leading a small resistance to stop him, but is captured by the Superman soldiers. Then we see the bit that premiered last night of their "leader" arriving.
I can't see this dream sequence lasting more than 4 minutes, and I'd lay down some money on a bet that this is NOT some vision of Wonder Woman's featuring other members of the Justice League.
I wrote this in another thread to illustrate the importance of the scene, as it relates specifically to the conflict between Batman and Superman:
He's afraid of what Superman COULD do and doesn't want to wait to find out. I don't think this particular nightmare is what inspires Bruce to act against Supes, and we don't know at which point of the film this sequence will come.
But this nightmare is essentially the manifestation of Bruce's greatest fear, which is being completely powerless to save someone or stop someone from doing something terrible. It all boils down to his parents' deaths in Crime Alley when he was a boy and completely powerless to stop their killer, Joe Chill. As Batman, he's trained himself to be able to tackle nearly any obstacle or opponent, and to make sure that he would never be powerless to save someone again.
Superman, however, is something that I'm sure he feels powerless against, having seen what he is and what he can do. He recognizes and admits that Superman possesses the power to wipe out the entire human race. In this case, the entire planet is Crime Alley and Superman is Joe Chill.
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I can even see the flashback of his parents' deaths and this sequence appearing almost side by side.