VLM traing goes large

We are a few weeks into my training for London Marathon and hit a bump in the road, figuratively speaking.
A few weeks ago I had my second 'jab' and within 30 minutes felt quite sick. It didn't come to anything but the next day it happened again, a general feeling of nausea.  On the Sunday i set out for my usual 10 mile plod and gradually felt worse and worse. I think a combination of heat and the jab were taking their toll and at 8 miles I gave up. Still 2 miles from home I had to walk on but felt very strange.
Over the next couple of weeks I limited myself to shorter runs and thankfully I'm back to a least 10k now without too much effort, although it has been very draining.
As I write, we are at the end of a very hot week in South Wales (31degrees) so running has been a bit more tricky. I had a great 5k last night with the weekly running group and plan a 10k early in the morning with a 10 mile Sunday to test how things have settled.
To cheer myself up during this hic-cup I have signed up for a further challenge, form the lovely people at End to End. This time taking on the original 874 miles of Lands end to John O Groats. With my marathon training this will help to give me a focus and a reason to get out even if it is for a few miles walking.
The online community for End to End are amazing - such great support, tips, general chat about running and challenges. This is now my third event with them and they have all been amazing.