Landmannalaugar is a popular spot for those wanting to do multiple day hikes, but there are also day hike options too. I’ve done Mt. Bláhnjúkur anti-clockwise for an epic elopement at sunset. Getting to the top is steep, but manageable with a lot of breaks. From the vehicle to the top of the hike it took us about 45 minutes; we were going quite fast though so I would say this would typically take 75 minutes.
It is a F road so you’ll require a 4×4 vehicle to get here. There is a river crossing you can attempt if you’re comfortable and wanting to park closer, but there is also parking before this river making it more accessible.