You are waiting for things to settle down first
The decision is not the part you are missing. You have worked out that you want to be running, and some version of how it would go is already in your head. What you are waiting on is conditions. Once this project is off your desk. After the holiday. When the evenings are lighter, or darker, depending on which way the year is going. You will start properly then, and the reason you are not starting now is that now is a bad week to be starting anything.
That reasoning holds up well, which is most of the trouble with it. Now probably is a bad week.
The week you are waiting for is described by what is missing from it
Look at how you have specified it. It is the week in which the thing currently taking up your attention is over. That is the whole description. There is nothing in there about what will be going on instead, because at the point you make the arrangement you only know what you want gone.
So the project finishes and you do not start, because by then somebody is ill, or the car needs sorting out, or work has handed you the next thing. None of that is unusual. That is what a week has in it. The settled stretch you are holding out for is not a period of your life that is scheduled to arrive. It would have to be a week in which nothing much happened, and weeks like that turn up too rarely to build anything on.
The start you are postponing is bigger than the one you need
It is worth looking at what you are actually proposing to begin. Usually it is not a run. It is a whole arrangement: four mornings a week, a plan with the weeks written out, the right shoes, a stretch of time in which you are someone who runs. That genuinely does need a clear month. You were right about that part.
The first thing does not. Going out once this week for twenty minutes asks almost nothing of your circumstances. It does not care whether the project is finished or whether anybody is ill. The gate you are standing behind was put there by the size of the beginning you had in mind, and you can take it down by starting something smaller than the thing you were planning.
Waiting does not feel like doing nothing
This is why it can run on so long. From the inside you are not somebody who has failed to start. You are somebody who has decided, and who is waiting for a sensible moment, which is a reasonable position to hold and even sounds responsible when you say it out loud. None of the discomfort of having given up is available to you, because nothing has been given up on. It has been scheduled.
What it costs you is quiet, and it is only time. A deferral you renew every few weeks makes no noise at all. You can hold the position for a year without ever having the day where you notice you have stopped, because you never started, so there is nothing to stop.
Start inside the week you have
So go out this week, in the middle of whatever is going on, and do a small amount of it badly. Twenty minutes. Some of it walking. That version will look nothing like the habit you designed, and what it gives you instead is information: how much fits into an ordinary week, with the ordinary amount of mess in it. You cannot get that from a week you are imagining, because the week you are imagining has none of the mess in it.
Something starting in a loud week has also cleared, straight away, the thing that would otherwise have finished it later. It can survive a loud week. It has already done it once. The habit begun in the calm stretch has to meet a normal fortnight eventually, and that is usually where it goes.
Pick the next three days rather than next month, because three days ahead is about as far as anybody can see with any accuracy. Decide which of those days you are going out on. Then decide what you will do on the worst of the three, the one where you get in late and nothing has gone your way, and make that version small enough that you would still be willing.
Nothing is going to settle down first. Things quieten for a bit and then fill up again, and whatever you want to be doing with your life has to be fitted around that rather than after it. A run goes into a week that already has too much in it. That was always all it had to do.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.