I run a couple 15 minute categories in two games but I want to try something really substantial. Any suggestions for games with long runs? Like 3-4 hours would be a good sweet-spot.

Depends entirely what sort of games you enjoy. The first thing I think of with longer runs is JRPGs, but many people don't like how those types of games play.

@Rukinom Xenoblade Chronicles 2 Mythra% - Estimate: 2 Hours.

Pokemon Crystal. Right in the middle of your sweet spot

https://www.speedrun.com/infamoussecondson (inFAMOUS: Second Son) Estimate: 3,5 hours

The first inFAMOUS is great, too. WR is a smidge under 3 hours but it's craaaaaaaaaazy optimal. I ran the game for months and months (returning SoonTM ) and my PB is a 3:26.