I thought their was no timer for story mode...huh... anyway the point is to get to lvl 20 as fast as possible while experiencing all the gameplay that you can. I can compare this to borderlands your first playthrough even with all teh side missions done oyu'll be about 35 to 40. the ndo your second playthrough you might hit 50 but alot of people only get to around 45 to 48. add all the DLC's and you'll finally get to lvl 50 on the first playthrough of 1 of them.
Story mode is exactly the same as all the multiplayer modes... The one thing you can change is who you play with and against. You want to play coop with humans instead of bots? Done. You want to play against human opponents? Done.
So far it seems like both campaigns follow the same series of quests. Res Q1 is Sec Q1, then Res Q2 is Sec Q2, with one being attacker and the other being defense, most likely alternating between the two positions on a nearly 50/50 basis...