The Fairfax County Cross County Trail is now more stroller accessible! I've never been on the County Trail north of Wakefield Park, but they have smoothed and paved large sections south of Lake Accotink down to Old Keene Mill Rd. If you don't mind some monster hills and stepping stone stream crossings the Cross County Trail is also greatly improved from Hooes Rd to Pohick Rd. The unpaved sections are as smooth as Burke Lake. This is great!
We have been running around Mercer Lake/Burke Lake for over three years now and though they are still the solid go-to runs, it is nice to have a little variety. I greatly prefer trails to roads. They are quieter, shadier, safer (away from traffic) and there is more nature to look at. The stream crossings are a great chance to play "Magic School Bus" when you turn into a sailboat or a motor boat.
Be prepared for the hills.