Again depending on how long you have been training, I believe you maintain your aerobic capacity for up to 8 weeks if you stop running or excercising, so although you maybe slower after a few weeks off, it shouldn't take too long to regain your original speed and heart rate etc.
Due to an injury problem I have not run consistently since April 2006, previous to that I was running about 5 marathons and 2 ultras a year, plus 20 mile races (as part of training), many half marathons and a few 10k's. Obviously I was training too..about 5 or 6 days a week.
Since April last year I have done nowhere near the amount of running per week as I used to. I don't even have a schedule...which I always used to have.
Some weeks I haven't even put my trainers on at all.
At the moment I guess I do on average somewhere between 10 and 18 miles in a week.
However, I've been racing some short distances as part of the Somerset Series and what I've found is that I can consistently run races at a pace quite a bit faster than I used to when I was running 5 or 6 days a week. In fact recently I got a 10 mile PB by 7 minutes!
Despite what is generally said, in my experience it takes a lot longer to lose fitness if you have a good base and take care of yourself in other ways, such as nutrition, sleep etc
I think if you've been in a routine for a fair time you don't lose too much over three weeks. Happened to me last year due to illness and I got back into the usual routine without too many problems. As Trinity says, if your starting from a good base you can cope with an enforced break ok.
I think you start to lose speed and muscle after about 1-2 weeks, although it takes a while longer than 3 weeks to go right back to the start, obviously it depends on how much you run. Also, if you do a load of running, stop for ages, then re-start again, you do seem to have a higher level of basic fitness when you restart (or at least, I did... :d )
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