。
Walking is good for your health.
Literal
Walk [nominalizer-のは] health [for-に] good thing is.
A combination of のは (nominalizer + topic) and ことだ (noun predicate) — 'the act of walking is a thing good for your health.' のは is the more conversational nominalization choice; ことだ as the predicate adds a declarative, almost aphoristic flavor, like a piece of received wisdom.