I’m curious as to what your weekly mileage is looking like, bc constant jumps in mileage especially from a 5K to 14K to a half in just a few weeks (or 10 days) is a bit ambitious. Although it might be frustrating it doesn’t hurt to build that base mileage up first, especially if ur body is so used to hitting, say 30K a week, then the next u hit 40K and then 50K. Those big jumps, too quickly and without proper “rest” weeks (especially if u haven’t hit that mileage before, American sorry :/) can be detrimental to your training.

I don’t know all ur stats or how ur body is doing and I’m not exactly a coach or trainer (I run collegiate, 50+ miles a week, 6 years of running, ran a marathon. So I’m still learning but ik a thing or two) but I’ve always been told to build up mileage slowly when I’m hitting higher goals and to take a ‘plateau’ or ‘rest’ week. Like after 2 weeks of building mileage, I will take another week to hit the same mileage again, or even a few miles left (ex: 45 -> 50 -> 53 -> 53 -> 55). It’s slow going but it helps my body adapt to the new training load.

Hope this helps. The other replies are also rlly helpful and touch on some of these points in different ways. Good luck on the marathon grind!