6 players in 4 hours. With 3 newbies (...) I see no problem to make the game shorter. Play only 3 campaigns, or just 3 turns in a campaign. (Have not tried this)
We did follow this suggestion (3 seasons x 3 rounds x 2 actions = 18 in all) and although it has some drawbacks (it makes the game a bit more static) it works as a fine 1st time check-out-the-game-mechanics type of play. That you will enjoy and can teach to your friends and still make a decent time.