Enjoy Rome
Contains a useful table, which details the average walking time between major tourist attractions in Rome such as the Colosseum and Pantheon.
http://www.enjoyrome.com/tourist/cityguide/public.transportation.html

In Italy.com
Try out one of the many guided and self-guided walking tours offered by this company.
http://www.initaly.com/ads/walking.htm

Frommer's
Features a selection of walking tours in Rome.
http://www.frommers.com/destinations/rome/0064010008.html

Virtual Tourist
Travellers offer helpful advice on crossing the road safely in Rome.
http://www.virtualtourist.com/vt/23513/8/f41e/