generally i don't calculate mileage so much as time i can allot for running.
I usually run for 40 minutes in the winter, sometimes more, sometimes less.
Some of it is on pavement, some of it is on packed snow shoe or skidoo trails, some flat, some in the mountain.So distance will vary a lot.
I check heart rate and pace and only check my distance once i'm finished which will be anything ebtween 6 and 10km .
But i can't say i'm in off season, running is only one of the sports i engage in.
January and February are quite dedicated to XC skiing and March and April are all about running. After that i split my time between running and biking.
August and September are all about the bike. Oct, Nov half and half, and then i'm back running only till there's enough snow to ski.
But i don't know about marathon runners . After some time off, don't you want to rebuild a good foundation for longer distances?