Stamina

[Gunther_01]

But first my thoughts on Personal Development.
The way I see it, what you learn should be all done behind the scenes, ie:after a couple of months of nonstop woodcutting, you realise that you are gathering more than what you have been normally.
In the case of construction, the speed improvements should be made within the item type ie Buildings, Tools, Machinery.
Skills could degrade over a long period of time.

Almost on the subject of personal development: stamina.
You have a stamina bar. It has a value (but you don't know what that value is). When you are travelling normally, the stamina bar remains full, but if you want to sprint (thieves would find a very good use for the sprinting part) the stamina bar begins to drain at a constant rate. When the stamina bar gets to empty then your speed should be very slow. As you keep moving at this slow gate, your stamina bar never fills again: you have to stop to regain your stamina.
The more that you run or walk, the larger the value of the stamina bar becomes.

[kasuo]

It should also apply to fighting/hunting. The more times you use a weapon in a day, the less strength you should be able to put into it as you get tired.

[Gunther_01]

Exactly, and thanks for replying.