Sheila is right, it will be so much harder on you than ZZero!
I was in the same position a few months ago, I returned to work after being at home with the birds for about 6 months. Did I worry, and still do! I am lucky I am staying on part time hours so I suppose I am quite lucky.
ZZero will be fine, just leave lots of foraging and chewable toys, leave some background noise on, I normally leave a radio on.
I think what Caitlin said is a good idea as well, start leaving the house for an hour or two now, try and get him used to this routine. The fact that you are starting on part time hours and building up to full time will help as well.
I hope you enjoy your new post and hopethat the change will go well for ZZero, Im sure it will
Good luck and keep us posted:)