Stanford study finds walking improves creativity (2014) 11 years ago
Perhaps a boost in creativity isn't solely restricted to walking but other activities which don't involve the task at hand (as others have mentioned)? For example, even doing a simple crossword puzzle to distract yourself may boost your creativity vs concentrating solely on trying to do something 'creative'. I highly recommend Professor Richard Wiseman's book: '59 Seconds: Think a little, change a lot' which references some studies involving topics similar to this.