March maundering.

[Sweder]

But that's because I've been ultra-cautious; running slow, taking walk breaks, and very gradually building up a base mileage, which still stands at a humble 25km or so per week.

Physician, heal thyself.

You've said it all right there mate. We have a talented bloke in our JSJ compliment - Paul the Goat - who runs every training run in search of PB pace. He's had to pull out of Paris because in his own words he won't break 3 hours. Paul is a numpty - he should listen to people like Sam 'the hobble' Lambourne or Lycra 'stitches' Tony; run your training runs like training runs, listen to your body, build pace and time on your feet gradually. Add MLCMan to those purveyors of wisdom

I have no doubt you have a 42k in you; it'll be one hell of a long road, but I have not a shred of doubt in me that you'll get there.