A good marathon plan will typically start with 4 weeks of hill training. Within those 4 weeks, the runner will work on three different hill workouts: hill sprints, medium hills, and long hills. I am always scoping out new hills in my long and easy runs, filing them away for the hill module to keep things interesting.
The Black Creek Greenway is my current favorite greenway to run on in the Wake County, NC area (otherwise known as the Triangle because of the three cities here: Raleigh, Durham, and Chapel Hill). This greenway stretches about 8 miles and includes multiple public parkings areas, various connections to other trails and parks, access to water, and even a Starbucks in case you need a little shot of caffeine to make it home.
This greenway begins at Bond Park in Cary, North Carolina and ends at Umstead Park just beyond Crabtree Lake at the Old Reedy Creek trailhead.