Consistency and the long game

This one is genuinely involved, and even on a coaching site the honest answer is that it is worth talking to a coach. A few things to check first: your consistency over months rather than weeks, whether recovery and sleep are really in place, and whether you are changing the stimulus or just grinding the same training. There are a lot of angles to look at, and having someone help you read your own situation makes a real difference.

Build running into your life properly, as part of the routine, and be disciplined about turning up. Life gets busy and things get in the way, so do not stress over hitting every single session, because that stress is the thing that makes it unsustainable. Over time you work out what a genuinely sustainable week looks like for you, and you protect it.

Rest. You have just put your body through a huge stressor, and it needs to recover properly so you can come back stronger for whatever is next. A couple of weeks off will not dent your fitness, and it does a lot for your recovery. A good rule of thumb is two weeks of nothing, or one week off then a week of cross-training, then a couple of weeks of easy running. Call it four quiet weeks, then build back from there.

Do the running you enjoy: a group run, a trail, a fun little local race. Keep it something you want to do rather than something you owe. There will always be days you cannot be bothered, but the test is whether it fits your life, and that is what keeps it going for years.

Definitely. 5 km, 10 km, halves, even a 1500, all good fun alongside the marathon work. Some help your marathon more than others, and a few are hard to do justice deep in a block, you will not run a lifetime-best 5 km mid-build, but they are worth keeping in for the enjoyment.