Failed Fartlek Training

Lately, I've only been doing long-distance training once every weekend,
so I thought I needed to adapt to race pace and tried a fartlek session.
A fartlek session consisting of 6 sets of 3000m/1000m at 4:10/4:35 pace.
While preparing for the marathon, I expected this session to be the toughest, and as expected, it was overwhelming.
My pace started slipping from the 3rd set, and my heart rate wouldn't come down during the recovery jogs.
It was such a disastrous failure that there's no point even feeling regretful.
I could make excuses like skipping the warm-up, having to use the bathroom because I drank last night, or my wife coming to watch, but
fundamentally, my aerobic capacity hasn't developed enough to handle that kind of pace yet. In short, a lack of skill.
I'm left with many thoughts today, wondering if I should give up on my goal of a sub-3 marathon.
I'll try to make up for it with other training sessions in the remaining time.
To all the M-Park runners, great job running through the holiday!