Well, it's done, and I feel amazing! Despite really looking forward to it, I was actually quite worried that I wouldn't keep going. Woke up at 6am thinking about it.
In reality, it was no problem at all. I actually think that for me it's more of a mental challenge than a physical one.
Anyhow, for those who are approaching this run with trepidation - don't. 5 weeks ago I was a proper couch potato with zero background in any kind of sport since I was (unsuccessfully) forced in high school. If I can do it, anyone can.