Will Road Biking give me similar levels of Cardio exercise to Running?

Don't worry,the most important thing to understand about exercise programs, is that they need to be enjoyable, it's the long-term habit that will change your life. On riding bike you want to ride like you are trying to make time, or make it somewhere on time. Coast when you need to, again keeping it enjoyable is everything and try standing while pedaling instead of sitting, the legs have to work much more. Always wear a helmet, the accident that can get you will most likely be the one you never see or even remember after a head-injury. Make sure you keep water, bike fix-a-flat, and a cell phone with you at all times when riding. I'd also keep a small set of tools, a few screw-drivers and a rachet, or wrench to take tires off as needed. Oh and a lock, in-case of a serious malfunction, or an accident that makes it too much to ride the bike for whatever reason, it can be locked up to a sign, tree or whatever, and save you the hassle of trying to get the bike home if problems arise. Bikes have the right of way on both sides of the street in either direction, but most people don't know this, so have a horn of some sort ( the electronic ones are good) and get the attention of anyone pulling out that might do so infront of you, I put my sunglasses up and make eye contact with the driver so I know I am good to go. Make sure you diet compliments your goals, count quality over quantity. And realize this is just another part of what should be a long-term habit of exercise. In time you might want to find others. But for now, enjoy! It's great exercise and the main way I use to lower my bodyfat percentage before doing shows. Oh and cycling is a lot easier on the joints, enjoy!

Top pro cyclists are among the most-fit athletes on Earth....

However, you need to do things right. First, I would recommend that you buy a heart-rate monitor. Cycling is an aerobic exercise and it's all about your heart rate. Read up on aerobic training zones and that sort of thing. The Livestrong site has a great deal of information on most all sports training.

Second, do some research on proper cycling practice. You want your bike to be properly fitted to you, properly set up, and of the right size. If you're riding a mountain bike for those distances, I submit you will be happier on a road bike. "If you want to go fast and far, get a roadster".

If you can't spring for a decent road bike, at least get some decent road tires for your mountain bike.

Learn how to use the gears properly. You want a smooth, high-cadence pedaling style.

Both will help your cardio but biking helps more for your stamina. neither of them will help eachother but running helps more in general while biking mostly is leg muscles and stamina. My advice is to do what you like but dont do biking instead of running as it is a much better general workout