Since it's non-Schengen to non-Schengen, you just wander over to the connecting gate and clear security there. The extra-(tough) security is only for U.S.-bound flights.

Trains run an average of every 10 minutes during the day, and I don't think it drops off much on weekends (check at www.ns.nl). Since your layover isn't all that long on the way back, you may not be familiar with Amsterdam and you are flying to the U.S., you might consider taking the organised tour (there's a booth between the base of the E and F piers, closer to the E pier). See what their schedules and costs are on your way out.

Otherwise figure about 10-35 minutes to get off the plane to landside (never figured if one could use the immigration post on the main level to get landside), 40 minutes to get a ticket and a train to Centraal, 2 hours in Amsterdam (check the canal tour boat schedules), 35 minutes to get back to Schiphol, and an hour to get back to the gate (bit too much time but just in case...) You really could have 4 hours in Amsterdam if all goes well (the trains aren't 100% reliable in winter) and know where you are going (as those of us who transit through AMS a lot do).