Winter blues?

Hi,
I tried to start my Shogun today after 3 days on no use.
I waited for the glow plug light to go out and then turned the key.
Unfortunatly the car would not fire, it was turning ok but not starting.
After about 15-20secs of trying, I turned the key back to off and tried again.
After doing so it started instantly.

Any idea why this is pls?, is the battery on its way out?. It does seem original and is now about 4yrs old.
I must say that the initial turning of the engine did sound a bit weak.

THanks
Amo
2002 Shogun 3.2 D-id