As far as I am concerned there are no short cuts or quick routes to fitness just as there are none to safe weight loss. Keep exercising and increasing your distance and/or speed and your fitness will improve.
I think the only exception to this is if you was fit and in shape at some point and you've not let it go tooooooo much, it's alot easier to regain, than if you'd never had it.
Reffered to as "Muscle Memory" I do believe.
IE I used to body build and was pretty darn big for my height, then suffered depression and couldn't be arsed, lost alot of it but stayed fairly healthy, not I am back on the fittness lark and the muscle mass is stacking back on alot faster than when I first started it years ago from scratch.