In this paper, a self-consistent numerical model is employed to investigate the ionization kinetics of the streamer-to-leader transition in long air gap discharges. The model focuses on the dynamics of the discharge channel in the radial direction using a 1D radial system by taking into account 21 species and 106 chemical reactions. The detailed dynamics of the streamer-to-leader transition after a sequence of streamer bursts, dark periods and aborted leaders are discussed. The calculations are performed for a 1 m long rod-plate configuration using the current measured in experiments reported in the literature. The results show that the average central temperature of a newly formed and a self-propagating stable leader segment is around 2000K and 3000K, respectively. The instant temperature in the gap as a leader segment is incepted can reach 5000K. The predictions of temperature and thermal radius of the leader channel agree well with previous experimental studies.