Then. Not merely temporal, as probably in Matthew 3:13, but almost consequential, "thereupon"; so also Matthew 3:15; Matthew 2:7, Matthew 2:16. John's preaching and manner of life were not without effect.
Went out; ἐξεπορεύετο (similar in the parallels). Our Lord, when referring to this (Matthew 11:7, Matthew 11:8, Matthew 11:9), uses the commoner ἐξήλθατε, merely indicating the crowds leaving for a while their present surroundings.
The synoptists here point rather to the trouble involved and the distance traversed. The singular is used (as often in the Hebrew) because the writer's first thought was of Jerusalem; the other parts were added as an afterthought.
All (cf. Matthew 8:34); i.e. from all parts and in large numbers. Judaea. Strictly speaking, this would, of course, include part of the next expression, but the reference here is especially to the hill-country.
And all the region round about Jordan; i.e. the inhabitants of the Ghor, the Jordan valley. They presumably came from either side of the river. "Strabo, concerning the plain bordering on Jordan, hath these words: It is a place of an hundred furlongs, all well watered, and full of dwellings" (John Lightfoot, 'Her.
Heb.').