For anyone interested or with the same issue:
Appart from James' solution, which I highly recommend, there is another formula (pandolf's):
Generate a slope (percent_rise), use the raster calculator and divide your slope in percent map by 100, giving you a dh/dx. Then use a formula {e.g. Pandolf's formula: M=1.5*W+2.0*(W+L)*((L/W)*(L/W))+η*(W+L)*((1.5*V)*(1.5*V)+0.35*VG) }*
Then you just have to covert KM per W to W per KM (1.0/output) and afterwards, if you want you can also convert to W per M (output2/1000)
*W=weight, L=external load in kg, V=walking speed, G=slope, η= terrain coefficient