I used to ride my bike every single day to college and back, that was 12.7 miles each way, 25.4 miles..., and man does it take alot to stick to that! that was like 5 hours of my day cycling...
i did it for 2 years and i went from around 22 stone in weight to around 11 stone in weight, now im a lazy bathplug who weighs 14 and a half stone lol!
That is by far the best exercise you can possibly get, theres no impact on your joints and it does some amazing muscles in your legs! lol