Now that I think about it, I really worked hard when I was in my thirties.
I would say that I worked a lot when I was in my thirties.
ご質問ありがとうございます。
① "Now that I think about it,"＝「今思ってみると、」
"I really worked hard when I was in my thirties."＝「30代のときはよく働いた。」
② "I would say that I worked a lot when I was in my thirties."＝「30代のときはたくさん働いた、と思う。」
ご参考までに。
I spent a lot of time at work from the age of 30 to 40.
この場合、次のような言い方ができますよ。
ーI worked a lot in my thirties.
「30代はよく働いた」
to work a lot で「よく働く」
in one's thirties で「30代に」
ーI spent a lot of time at work from the age of 30 to 40.
「30歳から40歳まで、仕事に多くの時間を費やした」＝「30代はよく働いた」とも言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
