つたない英語力ではありますが、読んでみました。

他の方が貼ってくれたリンク先を見てみると


If you departed by a commercial air or sea carrier (airlines or cruise ships), your departure from the U.S. can be independently verified, and it is not necessary to take any further action,

定期便（飛行機・船）でアメリカから出国していれば何もしなくてよい。


しかしながら、あなたの場合は徒歩での出国なので


If you departed by land, private vessel or private plane, you will need to take steps to correct the record. If you do not validate your timely departure from the United States, or, if you cannot reasonably prove you departed within the time frame given to you when you entered, the next time you apply for admission to the U.S., Customs and Border Protection (CBP) may conclude you remained in the U.S. beyond your authorized stay. If this happens, your visa may be subject to cancellation or you may be returned immediately to your foreign point of origin.


きちんとした証拠を提出して出国の事実を証明しなければならない。


To validate departure, CBP will consider a variety of information, including but not limited to:

Original boarding passes you used to depart another country, such as Canada, if you flew home from there;
Photocopies of entry or departure stamps in your passport indicating entry to another country after you departed the United States (you should copy all passport pages that are not completely blank, and include the biographical page containing your photograph); and


カナダから日本へ帰国した際の搭乗券や入国スタンプを写真にして
提出すればおそらくは大丈夫、ただし、considerが含まれているので
考慮されますが、確実ではないというふうに解釈しました。


To assist us in understanding the situation and correct your records quickly, please include an explanation letter in English. Your statement will not be acceptable without supporting evidence such as noted above. You must mail legible copies or original materials where possible. If you send original materials, you should retain a copy. CBP cannot return original materials after processing.


速やかに処理するために、英文の説明書に証拠を添付して送らねばならない。


We strongly urge you to keep a copy of what you send to DHS-CBP and carry it with you the next time you come to the United States in case the CBP Officer has any questions about your eligibility to enter. Carrying those materials with you will also allow your record to be corrected at the time of entry if, for some reason, the London, Kentucky office has not yet done so.


提出した証拠は、コピーを取って保管して、次回の入国時、質問があれば
提示できるようにしておいてください。


こんな感じの内容ではと解釈してみました。

https://help.cbp.gov/app/answers/detail/a_id/752/~/i-still-have-my-i-94