Great Falls National Park features 6 miles of hiking trails near Mclean, VA.
From I-495 exit 13 northwest of McLean take Route 193 (Georgetown Pike) west towards Great Falls. Go about four miles, then turn right on Old Dominion Drive, which becomes the park entrance. You will see signs for Difficult Run trail and parking on your left. You can park here for free or continue about 1 mile to the park entrance a pay a fee.
http://www.tripleblaze.com/trail.php?c=3&i=310