I cross country ski race both classic and skate along with backcountry skiing. Classic skiing is similar to skinning and would would be sport specific. Skating would give you fitness but not be as sport specific. The balance need to ski on narrow track skis and less supportive boots will help in the back country. I don’t know if you can skate on a split board but on bc skis I skate on flat approaches and flats on decents when the snow will support it. Track skiing either classic or skate when backcountry conditions are bad is good for fitness. Classic is easier than skate to keep heart rate down because you can walk with classic but there is a minimum energy output to keep skating.