Sounds like you're in a pretty similar situation to me - XC skiing is my primary sport, I usually ski 1500-2000k per winter. A couple years ago I realized a pattern in which I would get into really good shape by March and then gradually lose fitness throughout the summer, so I started running more seriously to maintain and improve my fitness between ski seasons. Last fall, I did a marathon in 3:22 after about 12 weeks of avg 25 mpw with 3-4 hours a week of rollerskiing or mountain biking. I'm now midway through a 16 week marathon training block, aiming for a 3:05 BQ time, which has slowly ramped up from not running this winter to 55 mpw, hoping to peak in the low 60's. I'm basically running 6-7 days a week, with one long run (15-18 miles) at the fast end of easy pace, one threshold interval workout, and easy runs with occasional strides or hills the other days. After my marathon in June, I'll bring some XC-specific training back in.

Anyway, if you look at what the pro xc skiers do in the offseason, there's a huge variety of cross-training. Yes, they do a fair amount of rollerskiing, but they also do quite a bit of trail running, both with and without poles. You encounter a lot more hills on trails, which mimics the natural intervals that you encounter while xc skiing through rolling terrain. This has the nice benefit of improving your running while also keeping your upperbody strong.

I think you would maintain your XC fitness just fine by taking the 18/55 plan and doing some of the easy runs as trail runs with poles. You could also substitute some easy rollerskiing for the rest days in the plan. Also adding some typical XC strength work (pushups, pullups, dips) would help you maintain that upper body strength. Skiing is so low impact that you could do your long run Saturday, followed by a long rollerski Sunday, and still be recovered enough for a running workout on Tuesday.

You should also look into the "Run Less, Run Faster" book as an alternative plan to Pfitz. It kind of gets downplayed on this sub because it's only 3 runs per week, but it has worked well for some who actually do the heavy cross-training that is prescribed in the book. I personally have found it helpful to look at the various plans (Pfitz, JD, Hanson's, Run Less Run Faster), and take the principles from each to make a more personalized plan that fits with my schedule and overall non-running training goals.