Hi Pablo,
First I think that what you're doing is a very familiar "way in" for many people. It's so natural for us to just lay down on the couch or in bed, and at some point in a lifetime of doing that, a number of people are going to experience something interesting there happening with the mind and consciousness. That was how the A&P occurred to me: laying on my back after a little cow-cat breathing in yoga and *boom* out of body experience that seemed to take "me" (personal awareness) right to the heart of God -- absolute homecoming, love. The Theravadan monk Yutadhammo describes how he trained in this energy (as a kid, way before he studied buddhist science of mind) while laying down on his back and an OOB experience he tripped; you can find that on his youtube channel.
I read in the MCTB about Three Doors to access Fruction, but clearly I've never been there, A&P is the highest zone I have stepped in. I haven't read so far any account of this doors to concentration, so this intrigues me.
If I understand you correctly, you're wondering if this recumbent position training (laying down and repeatedly getting into access concentration/early first jhana) will lead you into concentration training and the milestones of concentration training (the other jhanas)?
If that is your question, then yes. One of the four positions of meditation is recumbent on the right-hand side. This basically helps the mind stay alert against sleep. Recumbent position has uses, but there are times when it is counterproductive due to the ease of falling asleep.
The experience people have accidentally as their way into studying the mind deliberately (as in meditation) is usually so moving that they begin to search for re-creating that experience (as you are doing). This search to re-experience and understand what tripped the experience in the first place is concentration training: "Oh, what was that? I want that to happen again; what was I doing before when that happened?" Soon enough one is sitting on the floor or in a chair training just like one trains to play an instrument or at the gym.