Can I sub XC skiing for running? (Sometimes)

I’m scheduled for a long-ish run today but it has snowed about a foot over the last few days. I did a backcountry xc ski yesterday instead of running and I’m considering something like that again today in place of my long run. Am I short changing my training if I (classic) xc ski rather than post-hole for 8 miles? Would snowshoeing be a better alternative?

I’m near the end of a long 15 week transition period and I’m about to start increasing miles for the base period of the beginner 50k training plan. My longest run so far has been around 9 miles plus I’ve been sick a couple of weeks. At this point I’m beginning to be concerned that I haven’t put enough long runs in my legs and that a large bump in mileage in the next few weeks may put me at a higher risk of injury.

Last year I trained up for a 21 mile race but my quick increase in mileage gave me some knee pain. Trying to avoid that this year.