Sounds Bike you’re a new user?
When you set up your training plan for your A event, after you go through the plan builder you’ll get to the part where you can schedule the days of the week in which you ride
At the part you can also set your ride time caps for those days. On the day your long ride is scheduled click the drop down box and select a ride time cap that is close to what you want your long ride to be.
For example my plan has a long ride of 2:30 but I’ve set my cap at 4 hrs and plan builder has set my long ride closer to that mark as a result.