We would put a little more focus on what the marathon plan has for volume-intensity – especially during the last four weeks of the 16 week plan. It’s pretty important to taper for a marathon, and we would assume that the volume-intensity cycling on the CXC and your marathon plan don’t match up. What we would do so you don’t miss out on ski training is take a look at the two plans side-by-side and start looking for similarities.
A lot of running plans build in cross-training, so obviously, that would be a good time for rollerskiing. Swapping an occasional bounding or rollerski interval session for running intensity will be OK, too – maybe look at 1/3 to 1/4 intensity sessions swapped out for ski training intervals.
Sometimes, marathon plans put a lot of focus on mileage, and less on intervals – that’s fine, too. If your marathon plan is like that, we’d certainly make sure that we got the long runs in, and swap some of the early/mid-week stuff out for ski training.
The most important part, when you’re doing two sports simultaneously, is that you listen to your body and make sure you’re recovering well. We look at making sure you get the important sessions in – the long runs, the intervals, the technique – and if you need to back off, do it on some of the easy distance days.