You walked for a minute and decided it did not count
Somewhere in the middle of it you stopped running and started walking. Your breathing got ahead of you, or a hill turned up sooner than you remembered it. You walked for a minute, you picked it up again, and you got home. Then, on the way back, you quietly decided the whole thing had not really counted.
By the evening, that one minute of walking has taken up more room in your memory than the twenty minutes of running on either side of it.
The scoring starts while you are still out there
You feel it the moment you slow down. There is usually a glance around to see who is in sight, and then an attempt to make the walking look like it has a reason behind it. You get your phone out. You examine a shoelace that was perfectly fine, and then you walk on with a brisk, purposeful stride that is meant to communicate that this is a deliberate pause in an otherwise uninterrupted effort.
Then you start running again too early, before you have got anything back, because the walking feels like something to be kept short. So you set off into the rest of the run already behind, and it goes worse than it needed to. The minute of walking takes the blame for that as well.
Where you picked up the rule
Nobody ever sat you down and told you that a run has to be continuous to count. You absorbed it. Running gets described in unbroken units. Thirty minutes without stopping. Your first mile without walking. The language is everywhere and all of it points the same way.
School probably supplied the rest. Somebody sent a class out around a field, and by the far end the group had split into the ones still running and the ones walking, in full view, with everybody able to see which was which. That was a long time ago and it is still in there.
Your own plan carries it too. It says run for thirty minutes. It does not say be out for thirty minutes, which is almost certainly what you meant when you wrote it. Put the first way, any walking reads as an instruction you failed to follow.
What the walking actually did for you
Compare the run you had against the one that was genuinely on offer, rather than against some other version of that evening in which the hill was gentler and you floated over it. At the point your breathing got away from you, the real alternatives were turning round for home or standing still and concluding you were not fit enough for this yet. Walking brought the effort down to something you could carry, which is why you were still out there two minutes later, and still out there at the end.
It decides something about next time, too. A run you are allowed to walk in has somewhere to go when it gets hard. The all-or-nothing version has one move available, which is to abandon the session, and on a flat evening that is the move you will make. Over a few months that difference matters far more than the walking does. One version can absorb a bad day and the other cannot.
Decide where the walking goes before you need it
The useful change is to stop letting the walk be something that happens to you. Put it in before you leave. Walk the steep part of the hill and run from the top, or take a minute of walking in every ten and do not wait until you need it. You can walk the last stretch to your door deliberately as well, so that the run finishes somewhere you chose.
Walking you planned arrives as part of the design rather than as a verdict on you. It also means you walk while you still have something left, which works better than walking at the moment you have run out of road.
Nobody is keeping a column for how continuous it was. No version of your week marks the half hour down because some of it was spent walking, except the one you are keeping yourself, and you are free to close that. You went out, you were moving for thirty minutes, and you came home in a state that leaves Thursday possible. That was all the run had to do.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.