I also noticed that I became breathless quickly，and that I wasn't enjoying sport as much.

如果补充，也不是 as before。因为句子已经是过去时了，再用 before 时间出现矛盾。

其实，所缺的部分是说话人心目中与某种情况比较，但不必说出。也许跟另外一种爱好或活动比较。例如：I wasn't enjoying sport as much as music.

有时，as much 可以单独使用，表示程度。意思是“那么地”，等于 so much。