Can I enter a long endurance race that's further than my age group's limit?

Only in the open category, and without an age-group podium. Under section 12.4 of the Cycling SA Road Cycling Handbook 2026:

• Road races, time trials and team time trials longer than the maximum category distances are classed as endurance events, with no maximum distance.
• If an endurance event is longer than the maximum distance for your category, riders U19 and younger may ride it only in the Open category, and there are no podiums for their age categories.

Outside endurance events, the maximum race distances are strictly enforced (section 12.2).