I want to do a 2 day 50-60 mile hike in Joshua Tree this weekend or the next but I am unfamiliar with this park. Does anyone have any recommendations about a good loop or out and back?

I am far from an expert on JNP. I took one short overnight trip there on the Boy Scout Trail. You might try to locate a a copy of On Foot in Joshua Tree by Patty A. Furbush. There are no 50-60 mile hikes in that book. The longest extended backpacking trip mentioned is about 38 miles, but you might be able to link together a couple of the hikes to make something longer. Certain areas of the park (large areas it seemed to me) are "day use only" and restrict your campsite options. Also, the soft sand at the bottom of some of the dry washes looks inviting, but you wouldn't want to be washed away in a flash flood, so not all the "legal" sites are good choices if the weather isn't good. This was no problem to me, but might be nettlesome on a longer trip. It sounds like you are planning to cover a lot of ground quickly. JNP is not that huge, so you'll have to plan carefully. I found myself nearly done with my overnight hike way before bed time and had to occupy myself fiddling around and exploring this and that. 'Twas a full moon night and I could just as easily done the hike at night as in the daytime.

My trip report with Doug Johnson back in Feb lays this out pretty well. It was a one-way 40 miles on the CRHT (37.5) with cars staged at both ends and a water cache midway. 2nd time I've been backpacking on that trail.

Honestly, you don't need a book, just a map. Contact me directly anytime.

cheers,
-Michael