Thanks SAM and Katie, although the initial Backups are probably around the 100 gb mark per office I would imagine the deltas to be quite low in comparison about a couple 100 mb per site. so I guess as long as I spread the initial staging over different weekends for minimal disruption to the network it should work.
That's what we did, staggered over a few days for the initial and then just turned it loose.
With deltas like that, once you are set up it will be nothing to keep that backed up remotely.