Day 57/365 Not a good run

Day 57/365 Not a good run - Details
This morning I set out at 6 am to do 20 miles. Started of really good and about 12 miles in my legs started to feel weak which i found strange anyway i plugged on till the 15 mile mark at which point I had to throw the towel in as i was now feeling dizzy and for those who don't know that feeling dizzy for me can mean I could be about to get a shock from my ICD . So I walked the last bit back home and luckily i didn't get a shock and my heart rate was not near the danger zone but that small dizzy spell had knocked me. So here now i write about the first training fail. So far towards my marathon training I started in May I've done 10 miles,14 miles,16 miles, 18 miles and 15 miles besides various 10 and 5 km's.So I think despite today's small set back 'Im happy with the long runs I've done in prep for the Cheddar Marathon on september 9th.
 And don't think i'm only doing 1 mile runs till then ! I've got a 10k on 26 august to get a pb in
And I hope to get another long run in if I can free up some time