This is definitely much faster paced. There are different game modes and those tend to differ in average length. For example, a mode to kill x number of enemy demigods for a win isn't going to take as long on average as the destroy-the-citadel mode. There's no base building, no researching, so it's just jump in and go fight.
You can play co op vs AI online or LAN, but the auto-matchmaker for Skirmish and Pantheon does not currently allow to you make a preset team before looking, it just matches based on rankings, more or less. Custom games allow you to set up teams, maps, victory conditions, etc.