In the case of an tri event, almost certain the duration is just for the bike portion. And yes, I did that same ride a few years ago and it was 8 hours because I’m a slow climber (although Garmin has the 24 mile descent home at -3% as a personal best 40K 
).
That wizard will use duration and intensity to estimate TSS. You can manually override it too, here I’m going to set TSS to the ridiculous value of 999:
and that ridiculous TSS will show up in my training load:
Pretty sure the only impact of changing TSS is on the training load chart. In other words, changing event TSS from 50 to 800 won’t impact the plan you get from Plan Builder. The TSS is only used in your training load chart.