What bike are you bringing for your trip? You want to make sure your drivetrain is in good shape since there is a lot of black, volcanic sand up there in the highlands and it can wear out your chain and casette in no time. Same goes for rims; it works like using sandpaper for brake pads, especially if it's wet.
You can get temperatures ranging from -5 to +20 C - in the same day, winds up to 25m/s and horizontal rain. I've even had rain so bad that it got under the lid of my Ortlieb bar bag and got everything in there wet.
Be careful when crossing clacier-melt rivers as they tend to be brown and full of silt and moving stones. You can't really see the bottom so it's easy to misstep.
Keep in contact with someone and notify the staff of the various cabins around there, just in case something happens.
If you need anything else, just drop me a line.
Magnus Thor
Iceland