Thank you all for this insightful Running Power discussion, I’ve learned a lot about something I’ve not seriously considered up to now. Some further thoughts: What RP seems to provide is firstly a metric for accurately (& more importantly, consistently) measuring expended effort. By mapping this metric during training, it can be used to predict how much total effort is available at your current fitness level, and to optimally expend all that available effort during the course of a race. The second metric it provides is running efficiency and by mapping this during training it is possible to predict its relationship with terrain & fatigue and use that info to inform an appropriate race effort level. A third factor in efficiency (in addition to terrain & fatigue) is fuelling which is partly accounted for in running fatigue but needs to be managed carefully in long events to make sure you have available energy resources to meet the effort demands. It is generally accepted that to complete a course in the least amount of time, even effort or a slight negative split is optimal (all other things like terrain & weather & fuelling being equal); well trained & elite athletes are able to control their training environment so that they can accurately know what that effort is & feels like, and are able to monitor & adjust their effort when racing. For those of us who do not have that controlled preparation environment and have a less developed ‘feel’, RP provides us with that info on our wrists. With regards to flat vs hilly courses, efficiency losses will usually have a greater impact on hilly courses (uphill as well as downhill). Also, as others have said, it is sometimes not physically possible to run fast enough on the downhill to maintain the required effort level (RP). My understanding regarding wind resistance is that aero effects are negligible at slower than around 20km/hr (3:00min/km). But remember that this refers to velocity relative to the air, so at a running speed of 10km/hr (6:00min/km) a headwind of only 10km/hr (2.8m/s) will to start to have an impact, and this impact will be greater on the downhill when you are running faster to maintain uniform effort. So it can also impact overall performance (time) more on hilly vs flat courses, albeit to a lesser degree.