Lactate Treshold HR, advice

Hello, I’m a leisure athlete who trains alone (5/7 hours a week) I did this race on Sunday, 11km fast, I took a heart rate sensor, Garmin says my limit is 173, intervals.icu recommended 178, but I was more than 30 minutes above 181… can you give me your opinion, based on this race, to define my 1h heart rate limit?

Furthermore, I felt like I could continue like this for another 30 minutes, but my legs really didn’t want to respond, what should I train in this case?

Thanks!!!

link of activity

The most common field test is outlined in Joe Friel’s article: Joe Friel's Quick Guide to Setting Zones | TrainingPeaks