In this article, Walk Hard. Walk Easy. Repeat. by Gretchen Reynolds with NYtimes.com discusses the benefits of interval training. It is especially good for those that only are able to work out a few days a week and shown to improve cardiac fitness, reduce blood pressure and increase low-body strength.
For anyone unfamiliar interval training; essentially this workout method involves changing the pace, resistance and/or difficulty level of your chosen exercise alternating between low to moderate to difficult and back down. This article specifically highlights how a walking interval routine improved the physical capabilities for individuals age 44 to 78 in Japan.