You will be in a wide open area which is pretty trashy. Pedal up the small bluff to your left (west) and follow the ridge. You will find about 2 to 3 miles of really nice singletrack. Watch out for motorcyclist, ATV's and four wheelers in certain areas. This area has been used for decades by off-road vehicles. The Des Moines police have unofficially told cyclists they do not mind them back there (because people are less apt to dump trash etc.), but it may not be legal. I have ridden there for four years and never encountered a problem. If you are driving to the Brickyard it is best to park in Target's parking lot.
There are several ways to access this area, but the most common is to follow Martin Luther King Parkway North until it stops (This is just a few blocks North of the Euclid intersection in NW Des Moines). At the end of MLK you will see a dirt road (path) off to the right...just enter the Brickyard there by bike.
http://www.singletracks.com/item.php?c=1&i=127