Just to expand on Commander Mason's excellent solution above, here are the settings exactly as you need to enter them:
Choose Multiplayer>Xbox Live>Join/Ready>Player Match>Create Private Match
Mode: Co-op territory.
Type: Custom. USE THE FOLLOWING SETTINGS: Respawns: off, Base Zone: Base, Enemy Density: Infinite, Zone Failure: All Zones (I saved this as "Custom 01").
Time: 5 minutes
Advanced settings: Auto Launch timer: 30 seconds, Game Start timer: off
I'm about to start my third 6-hour run and wanted to post the exact settings for my, and anyone else's, quick reference.