That's a pretty good training plan.
There's been some research about sprint interval training (SIT), which seems to be popular these days, but low heart rate training (LHRT) is also popular.
Seems like your training was closer to LHRT, but after pushing yourself upfront, not as a sprint, but still initially unsustainably fast.
I'm a little over 40, male, out of shape, and I've been just running at 5mph. Much slower than your 7.5mph, but I can keep it up for 5k. (Side note: It's funny and annoying that treadmills in the US are almost entirely imperial even though most popular races are metric.) My plan was to slowly increase my speed, but now I'm considering your plan. It makes sense. It's easy to manage. It gets good results.