"This flight is too long."
Translation:Ten lot jest zbyt długi.
Not really. Your construction would only work if in the place of "lot" you put a unit of time: "Godzina to za długo", "Rok to za długo".
"Flight" by itself doesn't tell us anything about its length, therefore we describe it with an adjective to say that it is too long.
English word too has two main meanings (and few more less important ones):
(1) also; in addition – Polish też, również, także
(2) to an excessive degree; over; more than enough – Polish zbyt, za
If too is before an adjective, as in this sentence, then it means (2).
"zbyt" is the main answer that I see right now. Perhaps a bug made your answer rejected.