If I go to the gym, isn't there always a subtle motivation of making myself fitter / stronger? Does that motivation get in the way of oneness?
That is a great question.
You are right. It will get in the way if the self really wants something from it. If you workout because it makes you feel significant, or superior, or maybe you do it avoid some issue, as a reactive mechanism, then it is meaningless and distracting. But if you go into it, and while you are working out you are there completely, giving your attention to it, it is no different from meditation or anything else of value that you participate in.
When you enter the gym, be curious as to the outcome. You can guess the outcome, you can visualize the outcome, you can incant over the outcome, but you will only be there when you are actually there. If you approach it with curiosity you won't be disappointed in the outcome. You'll just change the course of action and go again. It becomes something that you are just doing. Over time, it might seem that these things just happen.
It seems easier to accomplish things from curiosity. It becomes, "I wonder if this effect will occur," instead of ,"Man, I have to do this." If you think you have to do it, you are projecting various things into the future. You may think of the result, but it will be overshadowed by the time it takes to get there. You will look forward to getting over it, as opposed to being there while you do it.
It is also a continuous movement, and never really hits a point of completion. That energy to find out is always there, pulsing as a kind of unstoppable wave that cannot find a conclusion. You may be able to see results, but there is never an end. It does not become, "I see. It is this way, and this way alone."
If you are curious, you have no idea if it will work. You are curious as to the outcome, and that may help you embrace whatever is there.
All you can do is move. Great patience may be needed.