We use run/walks a lot with folks we coach. Here’s how: First if the terrain dictates walking because it is too steep to keep HR in the desired zone then you should walk. Second if one does not have an extensive running background and wants to increase the length of the training session then mixing running and walking is a great way to to do this. Running of course if harder because of the impact and greater muscle mass involved but the training benefits of walking on steeper terrain will be even more sport specific for your intended goal. If you are very fit then walking on the flats may be too easy and not provide much training stimulus unless you adopt a race walking type of technique.