Excellent performance in Rotterdam, LR. Really well done. Sub-4 is great at your age...
Run-walk sounds as though it shouldn't work -- yet so many people prove otherwise. I like to quote Hal Higdon on the subject:
You will lose less time walking than you think. I once ran a 2:29 marathon, walking through every aid station. My son Kevin ran 2:18 and qualified for the Olympic Trials employing a similar strategy. And Bill Rodgers took four brief breaks (tying a shoe on one of them) while running 2:09 and winning the 1975 Boston Marathon. Walking gives your body a chance to rest, and you'll be able to continue running more comfortably.
The secret seems to be to do it from the start, even though it goes against the grain.
Is it just me? Or are marathons getting hotter, in the way that policemen are getting younger?