Jetlagged in Reykjavik

Most flights from the USA and Canada land early morning in Iceland. Roughly around 6AM
Depending on your transfer, you can expect to arrive at your accommodation about 2 hrs after landing time.
Check this post to read more about options on how to go from reykjavik to Keflavik and vice versa.

Once you arrive at your destination, which likely is a fully booked hotel, they will tell you that check in is later in the day, at 2pm or so.
In that case, its ideal to go for breakfast and aim to join a walking tour right after, probably around 10 or 11am and then after the walking tour, consider taking a brief nap. Its not recommended to fall into deep sleep that late and you might actually be better off staying awake until the evening and try to set the clock straight to Icelandic time.

If you have booked a pre-night or an early check in, you should try to catch some sleep maybe until midday (3-4hrs) and then you should be ready to roll for a walking tour at 1pm or so.

Of course this is very personal and possibly age will affect your jet lag. Never admit that age is the issue though. ;)