8 mile tempo run?

I think training for a Marathon based only on your 5k pace at this stage is a bit counter productive. I would just get out and concentrate on getting your distance up. Speed can come a lot later.

What is the longest distance you have done so far?

If you're not used to long distances you're risking injury.

Tim- it's my second marathon and I started my 16 week schedule with 14 miles last Sunday- no problems there.  I already do around 35-40 miles per week including one track session and one tempo run/ parkrun.  When I trained for my first mara in April I ended up doing seven 20mile+ long runs and actually ran the whole distance in training.  I'm planning to do the same this time.I love the endurance and have no issues putting in a lot of miles... it's the speed I seem to be struggling with.  I used my 5k time as this is my pb and my most recent (decent) race.  Like I said I've had a couple of crappy weeks but I'm hoping it's just a dip.