Hey, I’m Justin. I mainly have interest in running. Any ideas on where to start?

Hello! Firstly, welcome to speedrun community! Secondly, regarding your request, it depends on what game do you prefer to run / enjoy. Most of the time, i recommend speedrunning a game that you already played casually, so that you have an insight about the game (with added speedrun strats later on).

Other than that, i recommend speedrunning any short-duration games just to get the "feels" of it. Some games such as Minecraft (specifically, the short category ones; Minecraft (Classic) would be a good example), Roblox, any Webgames, or any Itch.io / RPGMaker games that exists on the SRC.