It took me several months to get there of pushing out my run distance a little bit longer day by day, but I was always on the treadmill for an hour. If I could only run for 20mins then next 40mins we’re walking. The walking pace was dictated by heart rate trying to keep my heart rate at around 150bpm. The long walks burned more fat than the runs and built out my leg muscles.
One I was able to get to a 50min 10K I found I was surprisingly close to a 20min 5k or a 12min 2mile. When I ran in a real 10k I did in under 48minutes on a course with many steep hills.
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.
If you want it on your preferred fitness device you'll need to use something it supports. Generally most places can import a GPX file https://en.wikipedia.org/wiki/GPS_Exchange_Format
They're not walling off features or asking for money, so I'm not sure if it's owned by another company or a FOSS project or something else. I did a quick search but it didn't turn up anything about their business structure.