Cycling 5.4 miles in 25 minutes on flat ground is hardly breaking a sweat.
The
secret to losing weight is to burn more calories then take in. Its that simple. So you need to figure out how many calories you normally take in and how many calories you normally burn and make sure the burn is higher.
This site:
http://k2.kirtland.cc.mi.us/~balbachl/calorie.htm
gives you some tables on the calories consumed from different actives. A 30 hour on a bike at 10Mph buns less that 300 calories. If you do some research you should be able to find figures on average calories for your foods and other activities.
I'm a active cyclist. I cycle with a club so I generally do rides in the 25-40 mile range at a pace of 10-14 mph. Since you live in NYC you might want to consider joining the Five Boro Bike Club:
www.5bbc.org
Which runs day trips on weekends in the city.
For me bicycling is more then just exercise. Its a way to get out and enjoy the outdoors, interact with people and keep my body in better shape.