3 miles in a treadmill does not feel like 3 miles on a track or park.

Try it yourself. There’s different muscles in my feet that get tired. I’m sure a treadmill is better than no training at all, but if you are aiming for a 5k try to do real walking or running.

There are certainly trade-offs, certainly if you have the option of walking in a diverse real location that would be better than a treadmill.

Different terrain, different elevations, not the same repetitive motion over and over again.