In reply to Removed UserGRUMPY MONKEY:
Most mountain roads won't be open until June and they'll close again in September, so summer in the Highlands is pretty short, as is the period when the interior is accessible by (expensive) public transport. http://www.road.is/travel-info/condition-and-opening-of-mountain-roads/
However, there's plenty of good walking to be done outside the months that the Highlands are accessble. It just takes a bit of imagination and planning - which is a decent proportion of the fun of a trip to Iceland.
Prepare for Scottish winter conditions and you'll have a good time, though you could get (un!)lucky: this week it's been warmer in Reykjavik than the UK. As phizz4 says, the Paddy Dillon book is a useful starting point.