I visit the forum regularly without defining an action to do so on my @Computer list. I'm simply interested in learning new ideas and helping others on their GTD journey. I could link these habits to 20,000 ft areas of focus such as self management and perhaps to 40-50,000 values such as contribution. There doesn't always have to be an explicit project between the higher altitudes and the runway.
The same could be said of an exercise program. Yes, you'd have a project to set up one, but once it's on cruise control it's a matter of keeping the habit and periodically setting new goals.