Hi - I never did a lot of intensity for running in marathon training. I would do a long run of 20 miles every second week which include 3 miles at MP at the end 2 or 3 times. The other 2 runs would be an 8 mile cut down run where I ran each mile quicker starting at an easy pace and finishing with 3 mile at MP/half/10k pace. The other run was a 2 mile jog after a turbo session. On the weeks I did no long run I would do 12 miles with the last 9 at MP plus a couple of 8-10 mile steady runs. The cycling was a zone 2 2 hour spin or 90 mins on the turbo plus an hour SS - maybe 2x20 or 3x15 at 88-90% plus an hour turbo with 40mins@ 80-85%. I included one road race in the build up where I ran 2:05 for a flat out 20m undulating accurate 20 mile race - eased back for it and ran it hard as I tried and succeed in winning the vets cat and finished top 10 overall. Overall however I felt it was the all round robustness I developed from the gym that helped me hold it together in the last 6 miles and also run a negative split…that and purposefully going out at exactly 20mins/5k pace all the way until 35k when I surprisingly managed to up the pace 
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