Yup, hold yourself back at the start, if there are lots of starters it's easy to get carried away, for every second faster than your ideal pace at the start you pay for it with interest at the end, easy to go off way too quick without realising. GPS watches, or footpod watches are very useful for keeping an eye on this. I went off way too fast on my first 2 HMs, only done 3 mind - but the third I managed to run every 5km in the same time give or take 20 sec and felt great at the end, rather than fast first 5K and very slow final 3 miles and be in pain over the line.