[Slick]

Use a bike. I can tell you from personal experience that walking is very boring compared to biking and you'll much more likely continue biking for the fun of it. I started biking for exercise over 15 years ago and continue till today.

BTW, you can't equate distances for walking/cycling. It's better to equate times - 20 min of excercise is 20 min of exercise. They aren't the same; personally I think you get a better workout on a bike anyway and you can taylor your speed/regiment. Plus if you are a beginner you get to rest a little on the downhills (and walk the bike up hills if you are a wuss).

What I did in the beginning is pick a close destination. I'd bike to it and back using different routes to minimize monotony. Then I bought one of those bike "speedometers". The thing measures lots of things: current speed, trip time, max speed, trip distance, total distance, cadence (pedal rotations per minute), average speed. I then would be determined to better my avg speed (and thus trip time) on every single run. It didn't matter if I beat my old time by 1 second, the goal was to do it a little faster each time. When I could easily complete the run at a pretty high avg speed, I changed my destination to a place a little farther away. Also, keeping cadence high (by use of different gears) is a key to efficiency.

I was in a great place for this since if I went one way from my house, I'd start with a long hill end up coming home with a long downhill. If I went the other direction it was the opposite. I called the hill on that side "heartbreak hill" since it was a gruelling climb at the end of the run.

I couldn't have kept my interest in exercise if it was just walking.