Hi Emma 
I reckon a real shocker of a long run during training is no bad thing - as Eggy says, it's good for the mental toughness....you know you got through it so you can do it again.
I had a biomechanical assessment at The Running School in Bristol today. With somebody who looked young enough to be my son....sigh.
Aaaaaaanyway, it was very interesting. I got videoed from the side and the back, then we picked it apart. There was more good stuff in there than I was expecting - for example, my alignment and arm movement is very good. I suspect this is a very recent change as I have spent a considerable amount of time and effort on this over the past couple of weeks. It seems that all my standing sideways on in front of a mirror rolling my eyes out of my head to see if my alignment looks right was worth it. I've stopped twisting my upper body, my arms stay in line where they used to cross over the midline. So far so good.
On the not so good, his main concern is my hip drop, meaning that my pelvis isn't stable. I'm not at all surprised about this, it's my weak area and it's what causes every single injury I've had, I think. My feet also cross over the midline too much but that could well be sorted with more pelvic stability.
I still heel strike but not as badly as I used too - that's the thing he's least concerned about.
Outcome is that these are things which can be sorted. My glutes are very weak (I'm not surprised by this either) so I have booked a session for when I get back from skiing. He's going to teach me how to properly do glute strengthening exercises, so I know how they should feel when I'm doing them. He's also going to help me with a couple of tweaks to get a tiny bit more heel lift, but he doesn't want to make too many big changes before Paris because my alignment is pretty good at the mo.
After that, we'll see...but today was certainly worthwhile as it's the first real feedback I've ever had about my running and it's told me that my suspicions that my hips were causing my problems were correct. And I do like to be right 