If you don't mind driving, the Assabet River Rail Trail is almost exactly 5 miles from end to end, you can park your car in Marlborough or at the East Hudson trail head and run out and back to your car. There's one spot where the trail disappears and you have to run on the sidewalk for a bit, but that's about it. If you don't want to do an out and back, there's plenty of sidewalk infrastructure around the trail, especially thru Hudson, if you want to branch off the trail.