Bear in mind that historical battles sometimes involved forces of over 100,000 on each side, and so a paltry 1000 isn't going to be that big of a deal.
Heh, I'm going to use rules from Tournaments, Fairs, & Taverns. This isn't a tournament, but the rules might work. Let's say that each competitor has a Combat Rating of 2, with a few higher-ranked guys having ratings as high as 10. If each army counts as one team, then each team would have a total rating of about 1000. 1200 max. So with a base DC of 10, going up by 2 each round or so, that's 496 rounds before one side or the other wins. Which would be about an hour and a half.
Now, take a baseball bat and try swacking at trees for an hour and a half, and tell me if you think that's not a long time for a fight to last. You'll be quite exhausted, and you're not even worrying about getting killed.