I'm not sure how this happened, but I made my mashed potatoes runny. Do you have suggestions on how to make it thick?

The quickest, easiest remedy to fix watery mashed potatoes is to add some dehydrated potatoes, a few tablespoons at a time until reaching desired thickness. Another simple method is to allow the runny mashed potatoes to remain on the stove, low to medium heat; stirring occasionally until the excess moisture reduces (evaporates) and the mashed potatoes are the desired thickness. Take care to not over-stir or the potato cells will break down and the mashed potatoes will reach an undesirable, ‘gluey’ texture.

Here’s a few more details:  https://idahopotato.com/dr-potato/how-to-thicken-mashed-potatoes

Don’t forget the gravy!

All Things Mashed Potatoes HERE