The reason you started will not be the reason you keep going
Ask anyone why they started and the answer comes back immediately. A photograph you did not like. A number on a form at the doctor's. A birthday with a zero in it, or clothes that stopped fitting. Whatever it was, it was specific enough to get you out of the door on a wet evening in the first fortnight, back when none of this was enjoyable yet.
Hold on to it, because it is doing real work. It will not do that work indefinitely, though, and almost nobody gets warned about what happens when it stops.
First reasons expire
The reasons people begin with are usually urgent, and urgency has a shelf life.
Sometimes the reason gets partly satisfied. You are fitter than you were, the trousers do up, the follow-up appointment went better than the one that frightened you, and the thing that had you lacing up at half six has stopped pressing. Sometimes it simply wears out. You cannot stay alarmed about the same thing for six months. The alarm fades whether or not anything about the situation changed.
Either way you end up in the same spot. One evening it is raining, and you notice you have no particular reason to go. You are not fixing anything urgent. Nothing bad happens if you skip it. The engine that carried you through the first stretch has gone cold without you noticing.
The gap is where people stop
A great many running habits end right there, and they end without any drama at all. No injury, no crisis, no decision. You go less often for a few weeks, and then not going becomes the arrangement.
What makes it worse is the story people tell about it afterwards: that they lost their motivation, as though it were a possession they had been careless with. Said that way it sounds like a defect in the person. It is closer to a supply problem. A temporary reason did what temporary reasons do, and nothing had turned up yet to take its place.
The part worth knowing is that the replacement does not arrive while you are sitting still. It comes out of the running itself, which means you have to keep going for a while with no good reason at all. That is unglamorous advice. It is also the whole trick.
What tends to take over
The second set of reasons is smaller than the first, and much harder to say out loud. People are almost embarrassed by them.
It is half an hour where nobody can reach you. It is the way an afternoon goes better after a morning run, or the way you sleep on the nights you have been out. It is that a Tuesday without it feels slightly wrong now, and you would rather not test how wrong. It is knowing you can cover a distance you could not cover in spring, and being quietly pleased about that whenever it crosses your mind.
None of that would look convincing written down. Nobody takes up running because Tuesdays feel wrong otherwise. But those reasons have a quality the first one lacked: they do not depend on results. They pay out on the day, every time, whether you were quick or slow and whether or not anything is improving on paper. That is why they survive the winter.
While you wait for it
If you are in the gap right now, and you can feel your first reason losing its grip, resist the urge to go looking for a bigger one. Bigger reasons burn faster. Shrink the ask instead and keep the appointment while it is boring. Three short runs you did not much care about are worth more to you at this stage than one heroic session in a week where you were mostly trying to reignite something.
And pay some attention to what you notice on the way home. That is usually where the next reason shows up, and it announces itself so quietly that it is easy to miss.
You are not less committed than you were in week two. You are further along than week two, and further along asks for a different kind of reason. Yours has not arrived yet. Go out tonight anyway, and let it catch you up.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.