You cannot walk for any reasonable length of time in road cycling shoes without experiencing massive discomfort. They just are not built for walking or casual wear. On the other hand, mountain biking shoes may be used for walking or casual wear given their more flexible soles as compared to road cycling shoes. However, even mountain biking shoes are not ideal for long distances. In general, it is best if you were to use walking shoes for walking and cycling shoes for cycling. Each shoe type has subtle differences keeping in view the different demands of the activity concerned.