Get as fit as you can.
It will have changed over the years of course but I'll bet the North airfield is still used as a major running course on IOT (lots of it!). We used to run from the hut on the edge of the woods near the firing range, thorugh the woods and over the horse jumps, around the woods and field back to the hut in 15 minutes (2.5 miles or so). And that was in combat kit with boots.
You can't be too fit. If you are fit, life's a breeze and in fact damned good fun!