Running slowly for a long time vs. running a bit harder for a shorter time

I used to jog for 20–30 minutes at 8 km/h on the treadmill at my apartment gym, but the weather has been so nice that I've been running outside for about two weeks now.
When I run about 5 km at my usual gym pace (8 km/h), I tend to stop around the 40-minute mark because it's too exhausting to continue, so I walk for an additional 20 minutes before heading back.
Today, I wanted to try running for a longer duration, so I lowered my speed and ended up running for 60 minutes, and I still have energy left. (I ran about 6.6 km in total)
Which one is a better workout? To be honest, I have no ambition to run marathons or anything like that. I just want to live a healthy life.