10 days of rest is for sure going to cause drop in blood volume. 10 days of very light training for someone who is in prime shape will result in poor aerobic efficiency after 5 days. Lactate will build at slower speeds, thus indicating that one will tire more easily at normal racing speeds.
Hypothetically, let's say Marius was in good shape and not overtrained with 11 days to go before his race. If he decides to cut his normal 25km per day of running to 10km per day, he will feel good at first, but by the end of the 10 days, he won't feel good after 1/3rd of the way through his race. He will have short-term power (snap in his stride) but a lack of endurance over the race duration. Instead of accumulating 8 mmols of lactate at 64 seconds per lap, he accumulates 10 mmols. Result? He hits max fatigue at 10 laps instead of 12.5 laps if he runs his normal 64 second pace.
If you don't get the lactate part, that is ok, because what really counts is the concept that it costs more energy to run at the same pace if you taper training too long. Fatigue accumulates quickly and you struggle to hold pace. No science is needed to understand that. We've all experience a loss of endurance; it's horrible! It doesn't matter how mentally tough you are: you simply run can't hold pace long enough in races. Tinman