Check out Skyline Drive in Front Royal. A little farther out than 30 minutes but you can get some incredible views and a near constant 15 mile descent if you drive into the park and run out. Might need someone to drive you back in.
http://www.mapmyride.com/us/front-royal-va/skyline-drive-route-1687466
I've done part of this run--over the bridge from Old Town, down to National Harbor until you can't go any further. There's a fabulous park in Old Town with a parking lot, bathrooms and water fountains. I was training for a half marathon so at the end of National Harbor I basically turned around and just kept going and it meets up with the MV trail again.
Here's the route I took. I had someone drop me off basically at the end of Duke street and said "See you in 2 hours!" The area that has parking is called Jones Point Park. http://connect.garmin.com/course/4874063
Hope that helps :)
Edit: Just remembered--the ONLY issue I had was with the gravel closer to National Harbor. It's basically crushed up shell and freaking horrendous to run on. Every single review I've read for the Wilson Bridge Half Marathon said that it's probably the worst part since it's the last bit of the race.