ok, well i've only done one decent distance race, which was 6 miles in about 45 minutes... the longer the race the slower people tend to go i think... I'm not particularly fast, but i'm not particularly slow either
as discussed in other threads, a lot of the exhaustion is mental, so training your brain is pretty important too. interval training helps with this.
i think the important bit is to be getting out and getting your heart rate up for at least 30 minutes, the rest will come with practice and experimentation.