The Fimmvörðuháls hike is a hike between Skógafoss and Þórsmörk, which is about 24km long. The hike takes about 10 hours, but you will be brought to the first location (Hvolsvöllur) and picked up at the end location, so all in all it takes a few hours extra. There is the option of getting a pick up in Reykjavík as well.

I have to mention that I have never done this hike, but I am definitely thinking about doing it some day because it looks amazing.