First the caveat: this is only my own experience. Based on that, a goal race in September is a long way off compared to getting in a great ski tour now. Assuming you have an overall solid fitness and endurance base, I don’t think transitioning to running and preparing for a race 6 months from now is a challenge.

Last winter we had a great, long winter and my last ski day (skijoring and some local ski randonnée) was 22 May, and similar to you I kept running in my training mix but skiing was more hours per week. Most important from my perspective was that I was maintaining the total hours training per week that I normally would do when running was my sole focus (10-12+ hr/week). From 23 May training was immediately almost 100% running (with some bikejoring mixed in) and 3 hour ski tours immediately became 3 hour runs. There was no transition problem because my legs were still used to running and I had the endurance/fitness base. In July and August I did a few 50 km training runs with a peak week of 154 km. Plans for a 50 km mountain race in late August did not quite pan out (pulled muscle on a cold day), but preparation was great. So, again, I would not think a September race should pose any problems for you.

My take is that structured training plans are fine as long as they accommodate some flexibility (which I suppose is the advantage of a coach).

Hmm, adjust … No matter what my dog still needed his exercise, so a morning ski was always on the menu. What the afternoon run would be depended on how I felt, but at some point during the week (starting in early April) I tried to get in some tempo, whether fartlek, intervals or progression. I should add my ski tours were solo, so I could keep treat them more like long runs.
At the moment with schooling in a second dog with skijoring and building his fitness, I have both dogs along on the runs and the tempo sessions aren’t possible (lots of snowshoe hares around, and also some reindeer — so the dogs have to be on leash). Eventually I can separate some runs from them to get back an interval session and a long run — and maybe a couple quick ski tours before winter runs out. But to come back to the question of adjusting plans, I keep a long-term view and flex the details as need be. For a race in September, I would think your 2-day tour would be good base-building — and fun.