When I was just mtb’ing at the weekend a full week of commuting or a lengthy road ride would leave me feeling run down. After doing more miles last year(in the region of 500 per month) I feel much more resilient and no more likely to get ill or anything.
Weather and daylight though make it difficult to get the miles in through winter. So to plan for that and build some fitness I would suggest 300,300,400,500,600,700,700,700,600,500,400,300 or similar.