Therefore (μεν ουν). Because of the preceding situation.

Was kept (ετηρειτο). Imperfect passive, continuously guarded, waiting for the feast to be over.

But prayer was made earnestly (προσευχη δε ην εκτενως γινομενη). Probably δε here is not adversative (but), merely parallel (and) as Page argues. It was a crisis for the Jerusalem church. James had been slain and Peter was to be the next victim. Hence "earnestly" (late adverb from εκτενης, strained, from εκτεινω, to stretch. In the N.T. only here, Luke 22:44; 1. Peter 1:22) prayer was

going up (γινομενη, present middle participle, periphrastic imperfect with ην). It looked like a desperate case for Peter. Hence the disciples prayed the more earnestly.