1. 这个句子，还有一个问题就是， I doubt 应视为插入成分。它跟虚拟主、从句表示的内容无关。

Had I started specializing in my sport at an earlier age, （I doubt ）I would have lasted for as long as I did or had the same level of success.

2. 在含有 should/were/had 的虚拟条件句中，可以省略 if,  然后将 should/were/had 放在主语之前，构成部分倒装。

网友的句子：

Had I started specializing in my sport at an earlier age, I doubt I would have lasted for as long as I did or had the same level of success.

= If I had started specializing in my sport at an earlier age, I doubt I would have lasted for as long as I did or had the same level of success.

又如：

Should he come tomorrow，I would tell you. 如果他明天来，我就告诉你。

Should the journeyer return, this stone would utter speech.“行人归来石应语。”

Were it to rain tomorrow, we would put off the basketball match. 如果明天下雨的话，我们将推迟举行篮球比赛。

Were she my friend，I would ask her for help. 如果她是我的朋友，我就会请她帮忙。

Had she studied harder, she would have passed the exam.　假如以前她努力学习的话，她就会考试及格了。

Had it not been for the captain，the ship would have sunk. 如果不是船长，船就沉没了。