Scott, your 5k and 5 mile times are quick compared to your 10k time and half time. You are quicker than me over those distances but I'm 1 and 2 mins faster over 10k and half respectively.
Off 22 miles now, I'd say you need to be building up some miles now. Your conversion up the distances gets weaker by each distance and it's no surprise looking at your mileage.
I did a 20miler 6 weeks out from London. My schedule had 20 miles at mp plus 15. I ran at at 7.07 pace. I then decided I would run London at 7mm, which ended at 7.04mm. Running relatively low mileage myself, I think you can get away with running a 20 miler a bit faster than someone on a 60 mile a week schedule. I didn't feel my 20 miler had an advrse effect, even on my next week's training.
well done on the pb, Dr Dan. I'm pleased that in part the First principle has contributed. I do think if you can improve your times at shorter paces it'll Carry through. Just need to add miles back in to convert more sharply.