Researchers say walking 100 steps/minute counts as “moderate” exercise:

http://www.medicalnewstoday.com/articles/142618.php

They recommend using a pedometer and making sure you get 3000 steps in 30 minutes.

Another way you can keep up the pace: walk to music! I use my iPod to train at my target pace, and you can, too! Here are 10 songs, 40 minutes of music, at 100 steps/minute:

1. Dancing Queen – ABBA – Pure Disco – 1976
2. Gone Going – Black Eyed Peas – iTunes Originals Black Eyed Peas – 2005
3. Don’t Tell Me – Madonna – Music – 2000
4. Fly – Sugar Ray – Floored – 1997
5. Europa And The Pirate Twins – Thomas Dolby – The Golden Age Of Wireless – 1983
6. When I Come Around – Green Day – Dookie – 1994
7. Life In One Day – Howard Jones – Dream Into Action – 1985
8. Welcome To The Jungle – Guns N’ Roses – Appetite For Destruction – 1987
9. Unwritten – Natasha Bedingfield – Unwritten – 2005
10. Genius of Love – Tom Tom Club – Tom Tom Club – 1981

I posted this playlist (minus Dancing Queen) on iTunes.