I live in Fairfax City and had a temporary job assignment near the Springfield Metro a coupld of summers ago. I took the Fairfax County Parkway Trail for most of the ride. I had to get creative bc the trail diverts and I had to cross the pkway at a light, then pick up the trail across the street. You want to pick up Hoos Road at the intersection of Olde Lantern Way/Gambrill Rd. Take Hoos Rd/trail down to Backlick. You want to take a left and not cross the street. There’s a pedestrian overpass which will shoot you out along Loisdale. Take a right on metropolitan Center Drive. About 100 yards on the left is a trail that will take you to the Metro. This trail is a bit sketchy, so watch out for glass. I wouldn’t take it after dark though.
Oh, the ride to the Metro is downhill, so your commute home is a bit more challenging.
Good luck!
Frank