To start of with I would recommend that you get a good pair of running shoes, I recommend Solomon speed cross 2 they are sturdy and have a good sole with plenty of grip.
Get a good running camelback or rug sack not too big but one that fits all the mandatory kit that is required for Gael Force 6.
Fit good tyres to your bike, ones that will hack the gravel and stones, Ideally Continental Gator skins or Specialized Armadillos size 23, Tyre pressure is very important, check the side of the tyre for recommended tyre pressure. A road racer is best, but a hybrid could also do the job. Make sure the bike is in good working order, probably best to get it serviced.