"cycling for weight loss" An efficient bicycle makes it very hard to burn calories. It is very easy to go long distances on a bike without doing much work. If you add a few bottles of gatorade you may even be behind in the cal buring.
"what kinda miles" You probably want a hilly route with little traffic so you can ride fast. To ride fast you basically have to keep 'accelerating' just to stay the same fast pace.
Consider using the bike for commuting or adding calorie buring to otherwise low calorie trips to the post office.
Consider a rowbike.com for an excersise bike. You might also put knobby tires on your mountain bike as a calorie burn enhancer.