Fit and Unfit
 

A year ago I was running regularly, getting to run 5 or 6 miles during my lunch hour with time for shower and changing.

Circumstances changed and I stopped the running, but I'm now ready to start again. I weigh the same as last year, look and feel the same, but when I go to run I can bearly manage a few hundred metres before wanting to stop.

Now that's the obvious part - you need to train and build up to running distances.

But my question is why? Is there anyone here who can explain in simple terms why we become 'unfit' when lacking in exercise? Is it the cardio-vascular system? Is it our muscles? The blood?

Muscle atrophy, heart/lungs, age, DIET, many factors.. such as.. how long you've been "slacking".
Basically once past your... mid-20s your levels of "ability" to maintain fitness drop dramatically. It also depends on your... "body type". Some can retain muscle and "fitness" better than others.
I have been a competitive swimmer until Uni times. But nowadays I rarely get to swim. I certainly can't expect to swim 1500m like I used to, or run 10km at the pace I used to, but with a little work and patience one can get to a pretty decent standard.

body aint used to it after a while, your muscles arent used to the exercising they used to go through a year a go

Muscular atrophy? Does that mean that the muscle tissue actually 'wastes' away so that there isn't as much remaining? That would make sense. So I guess if I restart training then overtime I should develop new muscle mass and this will help me to be able to run distances again.

Thanks for the replies.

Yep, simply put.. works like that ;)

EDIT: remember that training alone won't build muscle... proper "diet" is also necessary.. i.e. lean protein and complex "slow release" carbohydrates (bit of an over-simplistic view, but a good start I reckon...)

It's related to a number of factors:

1. Your heart. In very fit people, the chamber of the heart enlarges so it can pump more blood. This is called eccentric hypertrophy. (as opposed to concentric hypertophy - this is what happens in hypertension - the heart muscle size increases with a consequent reduction in heart chamber size - so the heart can't pump as much blood with each beat - and this is why high blood pressure can eventually result in heart failure). This process does not happen straight away. You will find that when you start running regularly, your heart rate will slow. This is because the heart can pump more blood with each beat, so doesn't have to beat as often. Take your resting heart rate now and in 2 months time and see if there is a difference.

2. The lungs. You need time for your lung capacity to increase.

3. Muscles. Using the muscles will cause them to hypertrophy (get bigger). You also increase the number of capillaries (small blood vessels) in the muscle with exercise so more blood (and therefore more oxygen and nutrients) can can to the muscles.

4. The blood. You produce more red blood cells with prolonged exercise. This is especially so if you exercise at altitude

Give it time. The body adjusts so that it can deal with its current demands. If you haven't been exercising for a long time, then changes will have occurred in the heart, lungs and muscles. This take time to change back. If you run 3-5x / week for 2 months I think you will be pleasantly suprised

BC
 

couldn't have put it any better myself

don't forget proper nutrition

& be sensible about resting when you need to, stretching properly and especially about decent trainers. I find cardiovascular fitness comes back very quickly, but the leg muscles take longer. Last thing you want is an injury so soon after getting back into it!

(PS - am not any kind of medic, just a runner who's learnt from experience!)

It has nothing to do with the blood or muscles or becoming unfit. The body becomes used to walking long distances and after a while when you dont train the body becomes used to not walking long distances.