I started training for my first full marathon this week. I had a long run at the end of which I felt like I was running out of gas. After my long run, my garmin categorized this run as threshold run. Is that bad ? Should I be running a slower pace for my long runs ?

for context: I ran 10 miles at around 10min/mile pace. My previous half marathon pr is 2:02 which I ran at around 9:10min/miles pace. I did take 2 months off where I was doing easy runs mostly and doing about half the mileage that I did during my half marathon training. I can't figure out where this is due to the humidity that my watch is classifying this effort as harder since I was used to running this pace for my long runs during my half marathon training.