I think the obvious answer would be to run OSRS.
Did you look at the page for OSRS speedruns? The average length is like an hour.
Best thing for you to do here is check out some speedrunning streams of various games you own/are familiar with and see if any of them look interesting to you. Taking random suggestions from strangers who don't really know you or your preferences rarely goes anywhere.
I agree about MMOs not really fitting for speedruns.
I didn't speedrun any RPG game yet, but if you are looking for RPGs, my favorites to casually play over the years are:
Games in the Final Fantasy series: https://www.speedrun.com/ff Games in the Kingdom Hearts series: https://www.speedrun.com/kh Games in the Aveyond series (Only the second game exist on the site right now: https://www.speedrun.com/aveyond2 ) Games in the Arc the Lad series: https://www.speedrun.com/arc_the_lad_series Games in the Mardek RPG series: https://www.speedrun.com/mardek_rpg
"Aveyond" and "Mardek RPG" series can be played on browser or downloaded for free.
Some games take very long time to complete; but some are quite short (up to 2 hours), or have shorter categories for completing a certain main task in the middle of the game.
If you are looking for other types of games, you will have to specify more about what you want to run or what interests you.