Watkins Park maybe. Not sure exactly how close that would be for you.

http://outdoors.pgparks.com/Sites/Watkins_Regional_Park.htm

Rosaryville State Park (about 20 min) is not far and home to some great trail running.

I'm pretty risk averse so I personally think you would need a death wish to ride 214 in the Largo area.

I'm from Maryland but closer towards Annapolis. I wouldn't ride 214. Most of the secondary roads connecting to it are probably fine though. If you don't mind driving a couple of minutes then take your pick of a loop anywhere south of route 50 down to North Beach. Just stay off of Route 4 and you won't have any problems. Beautiful area and would make for an awesome ride. Head down Route 2 and then turn back north and follow roads along the shore. Hit me up if you have a hard time figuring it out.

214 is ride-able, because it has a shoulder, but it's not something I'd want to be on for sustained riding. If you go a bit east towards Davidson you'll find lots of good roads to the south: Patuxent, Harwood, Owensville, Old Solomons, Rossback, all those sorts of roads are fine. If you're really ambitious you can ride towards Rose Haven and North Beach on the bay (farther east of those roads I described). I've only ridden this area on weekends but I'd think that after and before rush hours will be fine (between 9:20 am and 3:30 pm, e.g.).

I know of at least two teams that ride out that way a lot so I suspect a Strava heatmap will give you a lot to go on.

Another thumbs up for Rosaryville for running. I've done a couple of half-marathons out there and it's good trail running.