The first flush of (something) phrase
(at) A time when someone is young, strong, enthusiastic, etc., or when something has just started or is new or exciting.
Don't try to climb that mountain, dad. You're no longer in the first flush of youth.
He got too complacent in the first flush of success.
Everyone was excited in the first flush of the journey, but they all became tired as they got closer to the end.
The origin of this phrase is not clear.