Was Boris Johnson's decision to NOT run for Prime Minister of Great Britain a cowardly move?

This morning he announced he would not be standing for the prime minister position in the October elections. He was a big backer of Brexit, which surprisingly won.

Does Johnson not have the courage of his convictions? He supported Brexit, but does he now not have the spine to carry through with it? Did he really support Brexit, or was that just political Kabuki theater to make trouble for Cameron?

I see Johnson as a politician who utterly screwed up, and doesn’t want to take responsibility for what he did.