Viki (and Janie) - DON'T try to do too much too soon, or worry about pace or what you were doing pre-pregnancy. How soon you can start again will depend on the kind of birth you have and how you feel in the first few weeks after the little one arrives.
I had great intentions of being Superfit Supermum - back running within a week or two of the baby arriving - but the reality was rather different. I had an unexpected C-section which, looking back, really knocked the stuffing out of me. I tried to carry on as normal, not really resting much or allowing others to wait on me, and in retrospect I was really shattered. Started back running after about 6 weeks - despite being advised by the hospital to wait 4-5 months! I was shocked at how slow I was and how unfit I felt, especially as I'd managed to carry on doing a small amount of running until 32 weeks pregnant.
Rather than taking it slowly I launched straight back in to doing 4 runs of 3 miles each per week. This lasted two weeks before I strained my back. I don't know if I did it running or if I did it lifting Kit in his car seat and the running exacerbated it, but on the advice of an osteopath I stopped again completely for two weeks. Lesson learned, I built up much more gradually as soon as I started back again.
I completely forgot about what I was doing before (I ran a 3:23 marathon the month before I conceived) and went right back to basics following a beginners' schedule. I'm much better when I follow a schedule and am not tempted to do anything other than what is down in black and white. I've now been running again for four weeks and am happy to report no further back problems. You have to remember that the hormone relaxin that softens all your joints and ligaments in preparation for the baby's passage through the birth canal can remain in your body for about 6 months after the birth, making you much more susceptible to injury.
My first few runs were at around 9:30 minute miles, and now I'm able to run 7:45 pace once I'm well warmed up. I haven't pushed it though - I've let the pace return naturally as my fitness slowly returns.
My advice would be to take it slower than you think you need to. You have the rest of your life to be a runner again - why rush it?