Experiments in which several-meter-long infrared ultrashort laser pulses are guided in air are compared with numerical simulations. During a first self-focusing stage that is affected by modulational instability, the beam is shown to break up into two channels of light that finally coalesce into a narrow filament that is able to propagate over several Rayleigh lengths. The filament propagation is associated with the generation of an electron plasma, whose density is greater than 1016 cm−3. Electron generation persists well beyond the focal region. The simulations restore the global dynamics of the pulse, including the main stages of Kerr focusing, light guiding driven by ionization, and the ultimate diffraction of the beam.