If you are planning to do some hiking upon your visit in Iceland, we recommend reading this informative article by geophysicist Ari Trausti Guðmundsson:
There is a vast array of easy to moderate hikes to choose from and most towns, like Reykjavík, have a local “town mountain” 200 to 1,000 metres above sea level, which takes a short and leisurely one to four hours to top. For those with the skills, experience and physical endurance, there are also a number of more difficult mountains 500 to 2,100 metres above sea level. Hikers and mountaineers heading for these highest and/or remote peaks must be in excellent physical shape. Such day hikes can take at least 8–15 hours. Not to mention, weather conditions can be extremely difficult and seriously test a hiker’s endurance.