Impossible for me to know how fast you are, just saying that Sport and Super Vets are roughly comparable in terms of speed, and Open and Vets are fairly similar to each other too.
Unless you know someone who races, and can look at their past lap times in comparison with other categories, you’re best off picking what you think suits you best distance and convenience-wise. And if it turns out to be too easy or too hard, then move up or down a category next time.
Anyway, good luck and see you at the first round, Round 2 :)