Six miles north of the center of Boston, there is a nature reservation in which it is literally possible to get lost for hours. (Of course I do not speak from personal experience.) The Middlesex Fells Reservation is an amazing place to escape the hustle of the city and disappear into the woods. And even better, it is laced with trails which provide fantastic running opportunities. There are fire roads, which are usually gravel or hard dirt, fairly well-maintained, and wide enough for vehicles to easily pass through. Then there are mountain biking trails, horse-back riding trails, and even narrower paths that can really only be utilized by a person on foot. No chance of getting bored, especially when you get lost!