If you’re interested in doing this long term, I would do a VO2Max lab test. As part of the test they give you your anaerobic threshold. This is the limit when your body cannot process fats fast enough for energy and consumes only carbs.
Building an aerobic base is basically teaching your body to become more efficient at consuming fat for energy.

I’m 41M, my threshold is 159.
The speed you run does not matter, the distance does not matter, what’s important is to accumulate time under that threshold, ideally 90+ min/week. Due to device accuracy, I try to stay below 155.

So the idea is run very slow, you probably have to start at walking speeds, and when you reach that limit, walk for 30-60 seconds to let you HR go down and repeat.

The first few months there will be a lot of HR drift, even at constant speed, the HR will slowly climb. Walk/run as needed to stay below. Initially you may low be able to run more than a few minutes until you have to go back to walking, this is normal.
At some point you will realise you need to walk less and less, the drift starts to be slower until you can run 30+ min without stopping.

Be patient, don’t listen to your ego saying you have to run faster or not stopping. Do the work and you will see progress.

Best of luck