The more miles you can do without pain
I was a professional Triathlete and world ranked for a time being.
It took awhile, but you need to be acutely aware of the difference between pain and discomfort. Discomfort is something you can, and should push through. Pain is the warning of injury. Pain means you need to pull back and assess the situation. It's a fine line.
I applaud @Arf for how he advises you all on running. I know nothing about his selection, but his advice on running is golden for new long runners.