well i, myself, are no doctor or professionally trained at all with regards to the human body when it concerns health and fitness, but from personal experience, when i first started jogging i struggled a mile and after a while, being 2 months, i got up to about 8miles non stop - which was 2 jogs a week, 1 being 4, the other 8 ( 4- was fast paced, 8 slow paced )
I then stopped jogging for 2 weeks due to hurting my ankle at work. When i got back onto the jogs, i managed to pull off a 4 mile jog slow paced. So it didnt drop to the struggled one mile i started with but my fitness seemed to have halved. But i managed to get back to 8 miles within 2 weeks.
My dad, he is 52, and says that when he misses 1 jog in his week, then it takes him a very long time to get back to that stamina again.
but thats me, people might be different! But the best thing you can do with regards to this.... is get your running shoes on and run :d