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That's a bugger Paul. Not too worry...all the hard work is already 'in the bank'. Just need to get your back sorted and you'll be back on track :-)
Hmmm, it's all relative, isn't it? Your slowest pace on this workout was still faster than any per-km pace I've achieved in the last two months. I think it's your body's way of telling you that you've done enough work for the marathon and it's time to taper. As in, REALLY taper. Always smart to listen to your body ...
The only thing that's broken will be your PB in 3 weeks time.