I'm a mom who went form working 20 hours a week (4 days a week) for 5 years to full-time as of last October (two months ago). I COMPLETELY know where you are coming from, although I had to go back to work full-time for financial reasons. I put a query out on Mamapedia, and I had 30-plus responses from some fantastic moms on how they "do" it. You may want to take a peek...
I will tell you that the first four weeks of full-time work was absolute hell for me. I loved the job, but the additional hours took their toll on me emotionally and physically. Not to mention I was so sad to lose what time I had with our daughter.
We've had some challenges with our daughter; she hasn't enjoyed being in after-school care from kindergarten end (11:30 a.m.) to 5 p.m. and she has missed me terribly, but we've managed to smooth most of the kinks out with lots of candid, loving discussions in her bed at night as I put her to bed. Now we've settled down into a routine that works and I did get quite a few pointers from many moms on here, like I said. Just know that the first month or so WILL be challenging, but it will work out.
You have time, so prepare. Also, I don't know where you work, but you may check into working at a university. When I worked 20 hours a week, it was at my local university and that 20 hours a week also included retirement and full medical benefits.
Good luck, stay positive and message me if you'd like... :)