If you are going North on MD-355, at the northern edge of Gaithersburg, the road crosses a bridge over Seneca Creek. Just after that is a gravel parking lot and a trail head.
The trail between 355 and Watkins Mill Rd is about 3 miles, so about 6 miles round trip (according to the sign at the start of the trail, it is 2.9 miles). The trail doesn't follow the creek exactly, it is up on the side of a hill at the edge of the stream valley.
Many years ago I would go on this trail very frequently, and it used to be right on the edge of Seneca Creek for the first mile or two. They have relocated the trail further away from the creek.
I just took these pictures on November 10th, and you can see it is late fall and most of the leaves have fallen. There is still the occasional tree with green or yellow leaves and they really stand out against the other bare trees.
This isn't Seneca Creek, it is a very small stream that flows into Seneca creek. You can see the bridge where the trail crosses it. The trail is well maintained, and all stream crossings have bridges (or in one case, a huge rock). It can get muddy and the fallen leaves can be slippery when wet, so you do have to pay attention.
This area hasn't always been a park, and there is one spot where several cars were left to rust decades ago. There isn't a whole lot left of them now, but it is neat to see them out in the middle of the forest.