Tada, done as well 
Moving time 11:15, pauses 1h + 30min slow cruising detour, visited old school. Better than I hoped for (12h), even though power was lower – aimed for IF 0.66, managed to achieve IF 0.64, without detour might have been 0.65 
It was surprisingly easy, not harder than 200km preparation ride, just longer. Magic is in carbs. Usually for long Z2 rides I eat when hungry. This time planned it beforehand. Managed to consume 230kcal/h. Probably can train to handle more but for my usual rides this is enough.