Your heart is beating every second, whether you're riding on the bike or sitting on the couch. Your ancestors spent more than four hours a day doing hard physical labor. Unless you're seriously out of shape or have certain medical conditions, it won't endanger you any.

As for "extra fuel", don't start loading up. Most people consume far more calories than they need -- when they start to exercise, they don't need to automatically increase their calories, but they do often need to change the mix. Ride your bike, eat sensibly -- cut out the cheeseburgers and fries, stick to whole grain, fresh vegetables and fruits, and lean meat, in modest portions.