Forward is a World of Warcraft addon that allows you to toggle autorun using an ActionBook-powered action bar (such as OPie), or a simple macro command.
To get started, download and install the addon. You can then add the "Toggle Autorun" action to an OPie ring (find it under the Miscellaneous category), or use the following macro command to toggle autorun in any macro:
The video below shows how Forward can be used with an ActionBook-powered action bar and OPie. For integration with other action bars, use the macro described above.
Caveat: when Forward is installed, ending move-and-steer triggered by holding down both Left and Right mouse buttons will no longer end autorun. Binding-based move-and-steer is unaffected and will terminate autorun as usual.