You can hike from Jinshanling to Simatai easily in a day if you are in decent shape (I think it is about a 10-15km hike). Of the sections of the Great Wall I have been to (Badaling and Mutianyu), this is my favorite - much of it is still unrestored, so you are hiking on the original Great Wall, and there are nowhere near the number of tourists there, so you can even have a relatively "peaceful" Great Wall experience, which you will never get at Badaling (at least, this was the case in 2010 when I did the Jinshanling to Simatai hike).

The only annoying bit of the hike were a handful of poor locals who kept pestering us to guide us along a "short cut" off the Great Wall that was supposedly easier and would save us time. They would walk along with us, and keep telling us that the route was going to get so hard that we would never make it, and we should get off the path and let them guide us along an easy trail. Eventually I had to tell them to leave us alone.

It is a decent distance from Beijing, so you would want to leave as early as possible in the morning. I think the drive there took about 2-3 hours or so. The driver will drop you off at Jinshanling, and then you will meet them at Simatai at the end. The zip line down from Simatai is a pretty fun way to finish the hike.