Wii Fit was first revealed as Wii Health Pack during a conference in mid-September of 2006. Then described as a "way to help get families exercising together", the game idea had first been included in Miyamoto's original design document for the whole Wii Series, the entirety of which was scribbled onto a sheet of paper.
As with other games designed by Shigeru Miyamoto, such as Nintendogs, the design of Wii Fit was influenced by the activities in Miyamoto's daily life. Miyamoto states that he and his family had become more health-conscious, going to the gym and tracking their weight. He found that it had become "fun over time to talk about these things", and as weighing yourself "didn't make much of a game", they decided to build games around the idea to mesh with the concept.
In an interview with Game Informer at E3 2007 Miyamoto revealed that Wii Fit had been developed with a "full-scale" team for a year at the time. The Wii Balance Board had been worked on for "almost two years", and was inspired by sumo wrestlers' needing to weigh themselves with two scales.
Wii Fit uses a unique platform peripheral called the Wii Balance Board that can measure a user's weight and their center of gravity, and calculate their body mass index when told the user's height. The game has about 40 different activities, including yoga poses, push ups, and other exercises. Furthermore, Wii Fit allows its players to compare their fitness by using Wii Fit's own channel on the Wii Menu.
Wii Fit tracks a user's "Wii Fitness Age" through a daily body test basing the result on the user's current age, weight, and athletic ability.
According to Shigeru Miyamoto when speaking to IGN during an interview at E3 2007, there are currently no plans to integrate any WiiConnect24 functionality into Wii Fit. He did note, however, that there could be possibilities to take advantage of the WiiConnect24 in the future, such as using the service to keep in contact with a doctor to help with rehabilitation, or with a fitness specialist to help with training exercises.
What's it worth? Take a look at this Wii Fit price guide: sold listings for a value indication.