Physiological effects of 2-4 weeks of detraining
VO2 max: down 4-10%
Blood volume: down 5-10%
Heart rate: up 5-10%
Stroke volume: down 6-12%
Lactate threshold: Decreases
Muscle glycogen levels: down 20-30%
Aerobic enzyme activity: Decreases
Running economy: Unchanged
MY COMMENT: According to Pfitzinger, "most elements of your fitness go down at about the same rate at which they go up."
My theory, based on personal experience, is that after 3 or so days off, you need 3 days of training for every additional day you miss before regaining your original level of fitness. In other words, miss 10 days and you'll be back to normal in about 3 weeks.