Not really similar. The point of flâner is to let your mind go free, it is a relaxing experience that I could only compare to a massage. You just let everything go and just wander around, get lost in your thoughts, sometimes an external element cat he's your senses but it is purely curiosity and not really focus demanding. You are not exploring. You could do it in an IKEA for all that matters.
Going off trail trekking requires attention and focus, even if not constant. You can't just look at the sky and think about nothing if you risk fouling your ankle at every step or fall into a hole!
Off trail trekking is maybe more similar to coding something completely new. It's exciting, you experiment with different things, read bit here and there, and very satisfying, but also tiring. If I go flâner I will be less tired when I end than when I start.