One way to hold this word: your head has gone out in front of you and the rest of you is following, whether you like it or not. Once the head is leading, there is no foot on the ground to push back with — no brake, no turn, no second look. Every use of headlong keeps some part of that. Sometimes it is a real body falling. Sometimes it is a person moving too fast to see what is ahead. Sometimes nothing moves at all and it is a decision that has already left without you.
From the word’s history
The packet's source entry traces headlong to Middle English hedlong, an alteration of hedling / heedling / hevedlynge, later assimilated to the word long. So the second half was not originally long at all — it was an old adverb ending. The weight of the word has always sat on head, which is exactly where the picture above sits.
證據裡的來源把 headlong 追到中古英文 hedlong，說它是 hedling／heedling／hevedlynge 的變體，後來被同化成看起來像 long 的樣子。所以後半本來根本不是「長」，而是一個舊的副詞字尾。這個字的重量一直都在 head 上，剛好就是上面那個畫面的重點。
Wiktionary, "headlong"
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