You sound like you’re overtraining actually for your level and ability- it’s very similar to how I trained for my first marathon (also London, as a beginner!). Your mid week runs aren’t long enough or fast enough to sustain the kind of long run’s you’re aiming for at the weekend, but as your long runs are what’s wiping you out, it creates a cycle.
I’m also a slower runner (first marathon in around 6 hours, PB 5 hours 7 mins) but have now run 5. There’s absolutely no need for a 22 mile run, nor 2 x 20 milers if you’re not a super advanced runner. 5 marathons in and I still only ever cover 20 miles once in training, never more. I focus more around the 14 -16 mile mark. For my super long runs, I will attempt 2 x 16 miles, 2 x 18 miles and 1 x 20, generally in the pattern 16,16,18, drop back to 12 miles as a recovery week, 18,20, then into a taper. And generally one of those super long runs will go wrong or not get fitted in, in some way.
If you’ve done 20 already, I’d not try and go that far again, make your next couple of long runs 15/16 or cap at 3.5hours, there’s not much benefit to running over that in training, apart from the psychology of hitting the 20, which DOES help on marathon day- and look, you’ve already done it!
Maybe try and get something slightly longer in your legs mid week (maybe 10km) or some speedwork/intervals, as this will help make you stronger for the big day. Absolutely don’t shorten the taper! You won’t lose fitness from your months of training and you don’t want to feel that heaviness and tiredness you feel now on the start line- you want absolutely fresh legs- my longest run is always 3 weeks before race day. And yes the taper will make you twitchy, and paranoid (maranoia is absolutely a thing!), if you have a DP prepare them as I get grumpy as anything on a taper 😂. But once you hit the three weeks to go mark the aim of the game is fresh, rested legs and no injuries. Honestly, making it to the startline uninjured is a massive massive achievement!
I promise you if you’ve covered 20 miles, you will make it round. London is absolutely magical. The atmosphere is electric and when you see the 2 million people turning out to cheer you round, you won’t let them, or yourself down. When you’re finding it tough on the day, the crowd, the atmosphere, the other runners, something will pick you up and spur you on. They will still be there if you’re still out there at 3/4pm and you will forever be able to call yourself a marathoner!
20 miles with your legs.
6 with your heart.
.2 with your tears because the thing reduces me to a sobbing mess every single time!
Have the BEST day!!!!!