The key is to start slowly, but gradually increase distance and intensity. Also aim to look for hillier routes (mountainous best of all) to train on especially for the running, down hill running is as important as the up hill training.
If possible it would be a big advantage to have a look at the course especially the last third, which includes Croagh Patrick.
On the Bike, cycling skills are important as there are fast descent both on road and off road. GF 6 is hard with rough off road sections and hilly road sections.
See www.marreytrainingsystems.com for more tips.