Whatever best suits you mate. For me it depends on many things such as time constraints and what I'm doing before or after. If public transport to/from the event is an option like catching a train I'll try to use that. If I had work the previous day and got home late as a result I'd see if I could get dropped off so I could have a extra 20 or so minutes of sleep. Other events I'll ride 20km to the start, ride 100km and then ride back home.
I take it you live somewhere on the Gold Coast. Personally if I was you I'd drive to Carseldine Station to catch the chartered train to the start (as it will be a ~30-45 minute trip on the train depending on your starting point), complete the event, have a rest/something to eat/get some fluids into you and then take a slow relaxed ride back to Carseldine station (About 7km ~10-15 mins from the PCH depending on how fresh you legs feel
) to drive back home either though the city or via the gateway. I might be in that ride for the full 100km so I'll be riding to Carseldine station to get on the chartered train to Landsbrough station, pop over the road to the meeting point, ride the event and then ride the short distance back home.
Routes between the PCH-Carseldine Station. Both have dedicated bike lanes along Kittyhawke Drive and then a shared pathway from the lights to the roundabout.
Longer but less traffic/shared bike lanes and pathways route (can also take the Robinson Road-Kirby Road route to Zillmere road as there are shared bike lane markings and then cut through the old QUT up the road from the station using the shared pathway) - http://goo.gl/maps/CFpHB
Shorter but more traffic no shared bike lanes route - http://goo.gl/maps/tMd2x