回答
- I can't run as fast as I used to.
- I can't move as fast as I could before.
「足が遅くなる」はI can't run as fast as I used to.（昔のように早く走れない）I can't move as fast as I could before.（前ほど早く動けない）のように表現するのが自然かなと思います。ーI can't move as fast as I could before because I'm so out of shape.「運動不足で前のように早く動けない。」be out of shape で「運動不足である」ーI've slowed down a lot these days because I'm out of shape.「運動不足で最近かなり動きが鈍くなった。」slow down で「動きが鈍くなる・衰える」という意味もあります。ご参考まで！
カナダ 
