Erling Haaland and former Blue Mateo Kovacic struck for City's 2-0 win at Stamford Bridge.
Guardiola said: "I didn't expect it. We are away from our best so three points is the good news. The bad news is we have 65 games ahead of us.
"We have an advantage, it's our ninth season together so we know each other well. Of course there are mistakes you have to improve but we talked, the difference in being champions was our body language, how did we behave?
"It's the bad moments in the season and in the games that define the big teams. For me, it is an honour to be in charge of these guys. I'm so fortunate, I could not say more.
"There is a plan, defensive, offensive. But afterwards it's how we celebrate the good moments and come back from the bad moments.
"That's what defines these teams and no one talks about it. For a while we could never win at Stamford Bridge. This is a big step."