"the reservoir" in your context is an abbreviation people who know what it is (locals, people who know it as a film location) use. It's short for "The Central Park Reservoir" or "Jacqueline Kennedy Onassis."

Why is it called "reservoir?"

to be exact, it is called reservoir because it was originally used to supply water, kept in this huge pond/lake-like container, to parts of New York City, so it was not (just) an artificial pond/lake constructed for visitors of the park. It is officially called the Jacqueline Kennedy Onassis Reservoir, also known as the Central Park Reservoir.

In early scenes of the movie, Dustin Hoffman's character (Babe) is seen jogging around the reservoir.
The "empty" waterworks building later becomes an important locale for the pivotal confrontation between Szell (The Nazi-doctor who is a spitting image of real-life Nazi Dr. and monster Mengele) and Babe.

http://en.wikipedia.org/wiki/Marathon_Man_(film)
marathon man waterworks

http://en.wikipedia.org/wiki/Reservoir_(water)

http://en.wikipedia.org/wiki/Jacqueline_Kennedy_Onassis_Rese...

The Jacqueline Kennedy Onassis Reservoir, also known as the Central Park Reservoir, is a decommissioned reservoir in Central Park in the borough of Manhattan in New York City. The reservoir covers 106 acres (43 hectares) and holds over a billion gallons (4 million cubic meters) of water.[1] Though no longer used to distribute New York City's water supply, it provides water for the Pool[2] and the Harlem Meer, and it is a popular place of interest; there is a 1.577 mile[3] track around it for joggers, and it is also encircled by the bridle trail. It is often visited by tourists, especially when its double pink "Yoshino" cherries (Prunus x yedoensis),[4] followed by Prunus serrulata "Kwanzan" cherries, are blooming around the Reservoir.


http://gothamist.com/2007/07/31/central_park_re.php

Yay! For only the fourth time since it was finished in 1917, the fountain in the Central Park's Reservoir is flowing again. The reservoir, officially named the Jacqueline Kennedy Onassis Reservoir in 1994, served as the backdrop for Mayor Bloomberg as he announced that city water supply won't require filtering for at least 10 more years.

http://nyc.gov/html/dep/html/drinking_water/maplevels_wide.s...













--------------------------------------------------
Note added at 4 hrs (2008-02-19 04:56:24 GMT)
--------------------------------------------------

PS:

my answer should be:

(short for) jogging around the "Central Park Reservoir" or "Jacqueline Kennedy Onassis Reservoir"

--------------------------------------------------
Note added at 4 hrs (2008-02-19 05:21:44 GMT)
--------------------------------------------------

Or: "running" around the...