I rode 6/700 miles recently in normal trainers rather than my spd shoes in fairly close to brevet sort of times, (I have the same pedals as Machka has which has both options), and it was fine. I actually didn't have room in my minimal kit to fit a second pair of shoes or I would have had my spd's! As it happens, reading this forum today has made me realise I need to change my 14 year old mountain biking spds as I have problems with my feet when I wear them and think I need something better quality and stiffer!