I’ve done a few at Castle Combe which are good fun (if not a bit pricey now at £31!) but its a good mix of people competing, they are sealed surface events on the racetrack and perimeter road so fairly quick times, they run 2/10/2m distances so even biffs like me are done in well under an hour but its a really tough hour and you finish a physical wreck. All I would say is get in to trnasition and out as quickly as possible, no faff, no dawdling, no stopping for a breather, you waste so much time in transition its untrue, you think you are in and out quickly but its easy to waste two minutes there…
Also remember where you left you bike in the racks and watch out for the BTU officials patrolling the transition area who will DQ you if you touch your bike without first putting on a helmet, really….