Mike,
You gotta like to cycle that's for sure and if you are using cycling as opposed to running to get exercise... Chances are you will love it.
It can be addictive, especially if you are in shape.
My favorite ride is a 100 mile natural loop, it's also used for an annual bicycle race called the Tour Of The Unknown Coast. It's got every type of terrain and weather conditions most of the time. One minute you're coursing your way up a mountain in the hot sun, then you're getting an incredible adrenaline rush descending at speeds over 50 mph, then your cruising along a breathtaking ocean view and the next thing you know... you're up in the cool moist clouds. My average speeds for this course have ranged from 10 mph to 20 mph. Slower averages are from using a mountain bike and from wind conditions. There's an eight mile stretch right along the ocean heading North and if you hit a North wind... bye-bye average speed.
Some of my faster times can be atributed to being in good pace lines with other riders.
If you have a fairly flat ride and weather conditions are favorable... you should easily be able to average 16-18 mph if you're in decent shape.
Mike, I'm not recommending that you hop on your bike tomorrow morning at 4:00 AM. (You'd be calling your wife in no time to come pick you up to get you to work)
If you enjoy cycling and get yourself into shape, a 30 mile ride to and from work can be the perfect workout, to where you're not left craving to get more miles in each week.(IMHO)
I like to take the banana flavored Power Bars and Cyclo-Max for energy and hydration on rides overs 15 miles.
Tom, I usually gain back about 40-50% of my average mph's lost while climbing on my downhill descents.