Before starting marathon or ultra marathon training it is important to have reached a specific running ‘base’.  For me personally, I see 40km per week as an appropriate base from which to work.  You need to be comfortable with this weekly mileage.  This is base work (or your foundation) and I would say you need to be running this mileage for 4 to 6 weeks prior to moving onto specific marathon training.  Basically this equates to running 8km for 5 out of 7 days each week.  If you feel 5 out of 7 days is too much for you, you could always run 4 out of 7, with 4 runs of 10 km.  Your specific running permutation doesn’t need to be cast in stone. 

This base work is also the start of your mental and physical conditioning.  Physically these runs are conditioning your muscle and bone to the frequent stress of running. No matter which marathon training programme you choose to follow, you will need to have covered this base training.