18-20 miles run routes?

Well, your best bet is Central Park. You'll be in good company -- lots of runners all times of day. Outer loop is 6 miles, so you can do that three times. It's a pretty hilly course, there are public bathrooms at 80th street on the west side (the Delacorte Theater) and at 73rd on the east side (The Boathouse). Water fountains are all turned off, so bring your own fluids! 

http://www.theplaza.com/pdf/CentralParkRunningMap.pdf

(Sorry the map is a little nutty lookin', but you basically just follow the Central Park drive routes around the park -- closed to cars on weekends, so lots of room to run, just beware of the cyclists!)

If you want to do some more flat running, try the Hudson Greenway which is a trail that runs along the river, for pedestrians/cyclists/rollerbladers (might be a bit windy, but it's pleasant enough). The path goes along the piers in Chelsea, all the way up to the George Washington Bridge.

http://www.transalt.org/files/resources/mapwaterfront.html

You can combine them as well, if you prefer. Hope this helps!

The waterfront greenway looks pretty good. I was thinking about central park also but didn't want to deal with the hills since my race is completely flat. I probably should just htfu and do 3 laps at CP.

thanks for the info

Finally had some time to research run routes. Came across the site below and decided to plan for the bethpage bike path route. I'll add on to the 13miles to make it 18-20 depending on how I feel. 

http://www.lirunning.com/bethpage.htm