I am walking in both worlds of marathon and ultra, so my training (approx)
2 weeks of standard Marathon training
1 week with a 28-35 mile week end run
1 week with back to back long runs – anything from 20/20 to 40/20
1 recovery week with as much Marathon training as possible with recovery
Start the cycle again – I am flexible so this is just a general ideal.
I personally thing 1 weekend a month (out of 4 weeks) with back to back long runs is enough. But I am far from an expert.