How much time can you take off a marathon training scheudle without losing ground?

I have been training for a marathon for the last 9 months. My last long run was 17 miles. I have a foot injury and need to take a few dasy off - will I lose my endurance and progress and if so how long until I start losing it? Will biking help (such different muscle groups) ?|||I'm not sure but if you find out Id be interested as I haven't run for 2 weeks now.