From my experience,
When a mind is going toward jhana, it becomes centered and single-minded. It becomes very still. After the meditation this stillness will stay, so there won't be an arising of many memories and impressions in the mind. It is true that meditation can give you an insight into how active the mind actually is, but samadhi is stilling this activity, not making it (seem) worse.
What I think happened is you forced too much. I read "I tried to enter", "I stopped", "I started, doing". Being that involved can superficially seem calm the mind, but it does not really. That's why after you 'concentrated' and let loose, the mind was not at ease and it created many memories and impressions. In meditation you do not concentrate or use this kind of force. I think the translation 'concentration' is really bad as it gives rise to misunderstandings such as this. Instead you let the mind settle into peace and naturally all impressions fade away.
Sharp mindfulness also arises when the mind is calmed by not forcing it. It arises when there is little going on in the mind, not when there is a lot going on. So in a sense having mindfulness when the mind is restless is a contradiction. So what you have experienced in the walk may in some sense be connected to mindfulness, but not really be the strong version yet. It is more like clear understanding (sampajanna), although in my eyes that does not only include seeing things arise, but also know why they arise, if it is wholesome, and if not, what to do about it.
I hope this helps in a way.