Albert Barnes on John 6:21

Immediately. Quickly. Before a long time. How far they were from the land we know not, but there is no evidence that there was a miracle in the case. The word translated immediately does not of necessity imply that there was no interval of time, but that there was not a long interval. Thus in Mt 13:5, in the parable of the sower, "and forthwith (the same word in Greek) they sprung up," &c., Mk 4:17, Mt 24:29, 3Jn 1:14.