Perfectly OK to walk during runs. In terms of some of the training schedules you're expected to spend time doing recovery, which could be a light jog or walking.
Walk/run intervals is the best way to build up stamina in the beginning. As you get better you make the walk intervals shorter and the run intervals longer, and eventually you'll be able to run all the way.
Walking through drink stations to take in enough fluids to take down gels, helps to save time during a marathon. Avoiding GI issues, take in enough electrolytes/salt tabs to minimise muscle fatigue and adequate fluid intake to reduce the effects of dehydration.
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