Slow to interval

I wanted to do an easy longer run and started out at 9 minutes per kilometer. About 3,5K in, I started to feel my knee, I started to feel tired and there was burning grass in the road. It took me 10 minutes to get the story to the fire department. So I decided to just chuck it in and call it interval training instead. Did some ok sprints, but the bad start and perhaps the late hour had done their worst already. , Avg Cadence:71