Sister Baby's Buttermilk Pie

This is not a picture of the pie I made. I forgot to take a picture, so this is one I found online of the same recipe. My pie was not nearly so pretty. It browned to a dark brown on top, and the filling was still runny. It tasted good and all, but I am frustrated with my failures at custard pies. Every single one has turned out runny. I don't know if there's something wrong with my preparation, my oven, or the elevation, but I really want to figure out the problem. As I've researched potential solutions online, I've come across several options that I would consider cheating: adding cornstarch or tapioca to thicken things. I'm not saying it's morally wrong or anything (haha, the morality of pies), but I feel like the recipes were written that way for a reason and they should be able to work without adding an extra thickener.
So. Since I feel like I've become pretty good at pie crusts (which was my original goal in this endeavor) I think I will move on to making successful custard pies (which is differentiated from cream pies in that custard pies are baked in the oven, while cream pies have a filling that is precooked on the stovetop before putting in the oven). I won't necessarily be making a custard pie every time, but that is my new goal.
Here's a link to the recipe, which I would recommend for sure. And let me know if your custard sets well. I need some help. Oh, and the online recipe calls for orange zest, while my recipe in the book called for lemon. I think orange would be even better than lemon.

Ratings:

Flavor: 7

Execution: 6

Difficulty: Easy, if you ignore my custard problems.

Overall rating: *** 3 out of 5 stars.