If I remember rightly you being able to run about 3 miles in 20 mins is a good standard to aim for, but getting "hill fit" is really what its all about. During the course itself there is not a vast amount of fitness (3 sessions a week i think) due to the time available, so being fit before hand will help. If your up at sunny Kinloss then get out in the hills if you can carrying a bit of weight. Also will do you a world of good in the map reading/route planning aspect of the course. Other than that mate, go for it! I'm sure any of the lads/lasses on the crews will be able to help you out. Good luck!