Marathon preperation using Daniel's running formula

Why would you take four months off from running?

If you're in any kind of running shape now, then train for 16 weeks, and then take 15 weeks off, I think you'd be LUCKY to only end up where you started. I would bet a small amount of money you'd be worse off.

As the first guy responding pointed out, avoid taking a lot of time off running. You can do easy/casual mileage if you're between training cycles, but in general if you want to improve your running in a meaningful way, you need to stay consistent throughout most of the year.

As for your question #2, I wish I could give you a solid answer like "X % of what you averaged during the 16 week cycle". I'm not knowledgeable enough to tell you that. I know Daniels says it's easier to maintain a fitness level than it is to achieve it in the first place, but I don't know if he ever quantified it.

I do a lot of running on the treadmill during the winter. In fact, my mileage is often higher during that period. No reason to really need to cut back much if you have access to a treadmill & gym.

If you want to improve at running, you need to run. If you take 15 weeks off, you're going to lose any shape you're in. After a short break of 2/3 weeks say it should come back again pretty quickly, but after 15 weeks off you're not going to be in any running shape at all. In that sense the JD calculation is right - you will be more or less back to square one.

As the previous poster said, perhaps the answer is to get on the treadmill. Not ideal, and I wouldn't fancy it, but far far better than taking a 15-wk break.