。
Walking is good exercise.
Literal
Walk [nominalizer-こと] [topic-は] good exercise is (polite).
Notice こと in hiragana rather than the 事 kanji form — purely a stylistic choice. Japanese has flexible options for nominalization (の/こと/事) and the copula (だ/です), letting writers shift register and rhythm while keeping the core meaning the same. Here こと is preferred for the abstract/general claim about walking as exercise.