Missing a long run can feel much more serious than missing an easy weekday session. Long runs are an important part of marathon training, but one missed long run does not mean your race is ruined.
The first thing to avoid is panic. Do not automatically move the long run to the next day without checking what else is on your plan. A long run creates a large amount of training stress, so where you place it is important.
If you missed the run because of a simple scheduling problem and you are feeling well, you may be able to move it by a day. But only if doing that still leaves enough recovery before your next demanding session.
If moving the long run means you would run long on Monday and then do intervals on Tuesday, that is usually not a good trade. In that case, you may be better off moving the quality session, shortening one of the sessions or simply continuing with the week.
The reason you missed the long run is important to take note of too. If you were sick, unusually tired, sore or struggling with a niggle, the answer is not to force the long run in as soon as possible. Your body may need recovery more than it needs the kilometres.
You also do not need to add the missed distance to the following weekend. If your plan had 24 km this week and 28 km next week, turning the next run into 32 km to make up the difference is not how marathon training works. Your body responds to training load, not to a mileage debt that needs to be repaid.
A single missed long run will have very little impact on your overall fitness if the rest of your training has been consistent. The bigger issue is when long runs are repeatedly missed, especially as you get closer to race day. Then it may be necessary to adjust your expectations or your training plan rather than trying to catch up quickly.
If you have missed one long run, keep the focus on the weeks ahead. Get back into your normal rhythm, keep the easy days easy and protect your next key sessions. If you have missed two or three long runs because of illness, injury or a long break from training, do not simply jump back to where the plan says you should be.
Long runs are important because they help build endurance, confidence and the ability to handle time on your feet. But they work best when they are part of a sensible progression. Forcing one in at the wrong time can do more harm than missing it.
One long run does not make a marathon build, and one missed long run does not break it.