Louster - I have done the Chilly Duathlon. The run is mixed terrain and is around the outer perimeter of the race circuit,
The bike is 5 loops of the race circuit and so a hybrid is good. The race is newbie friendly and you will find people doing it on MTBs. One year a guy did it on a penny-farthing.
You cycle anti-clockwise around the race track and is flat, but you cycle into the wind on the second half of each loop.
Were you aware Chippenham Wheelers host an open evening at Castle Combe each Wednesday and everyone's welcome. Basically, pay ??2 to them and you can cycle the track as often as you like up to 8pm.
Sorry, I know you query was about the run and I have whittled on about the bike