Hahaha, I was shooting for 3hr but really 2hr was quite sufficient! I will see how else I can make up the "missing" hour to reach the magic 20....
(Went over to World Gym and did it on a bike in the spin studio there - it was very good, they are nice newish bikes and it is easier for me to stay focused when I'm out of the house.)
4 x 15min alternating 1:00 seated and 1:00 standing (high zone 2/some zone 3)
Now I am going to do half an hour of meditation and also my back stretching exercises - saying it here to make sure I actually do them - I think I will be sorry if I don't do them pretty much every day between now and my race, otherwise while running when tired I really have a tight sore spot where I pulled that muscle last year (spinal erector). Not acutely painful, but very tight and at its worst as though I have a sort of tingling phantom hot dog glued vertically onto my back!