Adam Clarke on Matthew 14:13

When Jesus heard of it, he departed thence - Had the blessed Jesus continued in that place, it is probable the hand of this impure female murderer would have been stretched out against him also: he withdrew, therefore, not through fear, but to teach his messengers rather to yield to the storm than expose themselves to destruction, where, from circumstances, the case is evidently hopeless.

The people - followed him on foot - πεζη, or, by land, which is a common acceptation of the word in the best Greek writers. See many examples in Kypke.