Accuracy of indoor runs is way off.

I am doing a lot of indoor runs but the accuracy is apalling. Today I did a 12k but the watch showed me 13.6k - that feels like a big descrepancy. Is there any way to edit what the actual distance of the run was? To also help with the future accuracy?

The only thing I can see that Apple recommends is to "calibrate" with outdoor runs - but my indoor stride length is shorter than my outdoor - so it doesn't help.

Is there a way that this could be implemented in future updates?