I came back from a 6 month lay off from an achilles injury by running half a mile and building up from there. Better to go slow and be safe, don't want to be out for another couple of months for the sake of pushing yourself.
In all honesty I think it is really hard to predict your time at this stage (although you can have a ball park idea).
I was looking to get just under 4 hours in my first with a 1.42 half a few months before, but an injury put me back a bit then the training later on accelerated faster than expected then again I wasn't quite fit when the day came. I did a 3.55 and was ecstatic with that, but I have always had a little nag that I was quicker than that when I did my last long run.
So it can ebb and flow quite a lot as training moves along....