This trail is not typically used by runners and is more heavily used by mountain bikers. Runners tend to favor other trails in the park as this one is commonly very muddy and can be a bit of challenge if you're looking to keep your feet dry.
Start at the small parking lot on the south side of 28 (1/2 of a block west of 28 and Dufief Dr.). The trail stays pretty much level along the Muddy Branch Creek and is well marked. The trail crosses Quince Orchard Road and then Turkey Foot Rd, and upon meeting Esworthy, you have to run along that road (in a northwest direction) for about 4 blocks to pick the trail up again off to the left. The trail keeps following the Muddy Branch Creek, with more ups and downs in places until it reaches Blockhouse Point Park. From there, run down to C&O Canal parking area at Pennyfield Lock Rd.
In all, this run encompasses an almost 9 mile journey on mostly singletrack in the thick of Gaithersburg/Potomac. It's popular with mountain bikers though so be aware of your surroundings and of other users on the road.