It's certainly a nice place to visit. A couple days for the Snaefellnes peninsula would seem appropriate. We did most of it in a day, but hurried due to time constraints. If you want to take your time and enjoy it, plan on at least two days. The Golden Circle (inland) is a trip to be considered and I believe there are plenty of day tours available for that area. We rented a car and did a self tour, thus I can't recommend a tour company. Time permitting, you should consider heading to Vik, but my preference would be the above two locations first.
Don't forget the Blue Lagoon. It isn't far from Reykjavik and tour companies run there daily. I'm not into that stuff, but clearly many people are.
We purchased a map and e-book from the International Photographer for planning our trip, which I noticed from another post here. It is an excellent, high quality map that shows all the iconic picture locations and was used regularly the whole trip. I'm sure there are other great guides available as well.
The good news is that I didn't find a lot of stamina was required for the majority of scenic locations. Fairly easy access and reasonably short hikes. Good luck!
Miles