Take a look at the original sentence again:
Wally did not want to do anything to ruin his new pair of athletic
shoes, he always stayed on the sidewalks,
sidestepped puddles, and made several trips a day to the bathroom to dust
the sneakers with toilet paper.
You chose to fix it the correct way:
Wally did not want to do anything to ruin his new pair of athletic
shoes, so he always stayed on the sidewalks,
sidestepped puddles, and made several trips a day to the bathroom to dust
the sneakers with toilet paper.
A perfectly legal move in writing is using a comma and coordinating conjunction like so to connect two main clauses. So shows result; thus, it works perfectly in the example above.