It's surprisingly easy when you're there - I had exatly the same problem!
Start off really really slow, just a really slow jog and keep a steady pace. Everyone around you sort of spurs you on, and there are people standing at the side of the road keeping an eye on things and encouraging.
That's what i did, then when I saw the finish line at about 200m or something, I just legged it, never run so fast in my life - finished it in 36 mins! My record in the gym was never under 40! Just go for it, keep a steady pace and good luck! xxx