'Walk', meaning an excursion on foot, is a countable noun.
This means that it needs an article when used in the singular. You could say, as Jmat suggests, 'Walks are good for you' or, for example 'A walk every morning is good for you'.
Saying 'Walk is good for you' is just as wrong as saying 'Friend is good for you' - it just sound like 'pidgin English'. Singular countable nouns can't stand alone.