This is an unmarked multi-use trail that's especially good to ride in the morning when the sun is rising over the trees and shines onto the reservoir. Just watch out for equestrian riders and dogs being walked off-leash (I'm unsure about the leash laws, but you'll certainly encounter them).
There is parking right on Deer Park Road. From the trailhead, follow the main fire trail and at about a quarter of a mile take a right at the first trail split. You'll come across plenty of fisherman trails branching off to dead ends. It can be frustrating to accidentally following one, but you won't get lost.
Continue on the main trail to a creek called Middle Run
. You'll cross the creek (high erosion here), and take a left on the other side.
Stay on the main trail that hugs the shoreline of the reservoir (the trail to the left of each intersection). Just after two miles, the trail will split, one headed to the reservoir and one headed up a hill. Head up the hill. By 2.6 miles, you should be able to see you are at a peninsula. Follow the trail till it ends and head back!
Java Mama's in Reisterstown has a great breakfast or try Harryman House for a good brew depending on when you go to the trail.