As John was fulfilling his course (hōs eplērou Ianēs ton dromon). Imperfect active of plēroō, describing his vivid ministry without defining the precise period when John asked the question. Paul uses this word dromos (course) of his own race (Act 20:24; 2Ti 4:7).
What suppose ye that I am? (Ti eme huponoeite einai̇) Note tōi (neuter), not tina (masculine), what not who, character, not identity. It is indirect discourse (the infinitive einai and the accusative of general reference). Huponoeō (hupo, noeō) is to think secretly, to suspect, to conjecture.
I am not he (ouk eimi egō). These precise words are not given in the Gospels, but the idea is the same as the disclaimers by the Baptist in Joh 1:19-27 (cf. also Mat 3:11; Mar 1:7; Luk 3:16). Paul had a true grasp of the message of the Baptist. He uses the very form lūsai (first aorist active infinitive of luō) found in Mar 1:7; Luk 3:16 and the word for shoes (hupodēma, singular) in all three. His quotation is remarkably true to the words in the Synoptic Gospels. How did Paul get hold of the words of the Baptist so clearly?