It's reversible if someone is willing to do the work required. You'd have to sand off the old stain (and any finish product) and replace it with new stain or paint.
If you feel yourself getting tired, lower your pace to about half as fast as you were going instead of giving up. Run just fast enough to make it the...
You keep going. Motivation is key, as if you allow your body to stop, it will just become even harder. Since it's about 1/2 a mile, it is not recommended...