fall in step

Kazuo Ishiguroの "The Unconsoled" を読み始めました。Kazuo Ishiguroの小説を読むのはこれで10冊目で、地元の図書館にある氏の小説はこれで全てです。

'Where is it Boris wants to go?' I asked his mother quietly.
'To the swing park, of course.' Sophie sighed and rose to her feet. 'He wants to show you the
progress he's made.'
There seemed no choice but for me to rise also, and the next moment the three of us were setting
off across the square.
'So,' I said to Boris as he fell in step beside me, 'you're going to show me a few things.'

"fell in step" を現在形の "fall in step" で辞書を見ます。
"fall in step" では辞書の項目として見当たりませんでしたが、 "in step" に次の説明があります。

・Collins Dictionary: marching, dancing, etc, in conformity with a specified pace or moving in unison with others
・Cambridge English Dictionary: When people walk in step, they lift their feet off the ground and put them down again at the same time: The soldiers marched in step.

"fall in step" は歩調を合わせると解釈して間違いないでしょう。
イデオム辞書のFarlex Dictionary of Idiomsに "fall into step" について次の説明がありました。
To begin walking at the same pace as someone else.: If you don't fall into step with us, you'll get left behind.