Choose whichever you think you will ride the most.
Bascically, there is no majic pill, it will take time, time on a bike, time running, or time doing some type of cardio exercise.
If you could ride 3-5 days a week you will see vast improvement very quickly. One option is to by a MTB and and extra set of wheels with road type tires on it. Then you could ride both at anytime.