Running to skiing should be a 1:1 ratio and your HR zones should be pretty close to the same. The major difference between running and skiing is that in skiing you are at less of a steady state with your heart rate than running. Instead, when skiing, your heart rate will vary more as the terrain changes. When running, we do not have the same recovery on downhills where we stop working to move forward. The only caution I would give for substituting running in place of skiing is be careful not to drift out of your zone into a higher zone than you want to be at, because you will not have the same type of rest/recovery on the downhills.