This is sort of how I made myself sign up to the trek in the first place - if I put it in writing and involve someone else, then I am committed. So, lets see if this works:
the running (NHS Couch to 5K) plan is going so well that I am almost a week ahead of schedule. I started it at the beginning of the month and it's now the 21st and I am almost finished week 4. This is so promising, and I don't hate it yet that I thought I'd plan to get ahead of the schedule and write it. This will accelerate my fitness and get me running 5K and more (?) before Christmas.
w/c 21st - finish the program wk 4 and wk 5
w/c 28th - finish wk 6 and start wk 7
w/c 5th - finish wk 7 and start wk 8
w/c 12th - finish week 8 and start wk 9
w/c 19th - finish wk 9 and TA DA! I am running 5K!
w/c 26th (aside from the hangovers) - more 5K
then just carry on running 3 times a week until the trek... and after!