Missiles already glide. Most air-to-air missiles fire for only a little bit right after they're fired, then spend the rest of the flight coasting. Long range missiles propel themselves on a lofted trajectory to preserve energy, falling on their targets from above. In general, missiles can burn energy very quickly by making high-G turns, but can't recover energy, while aircraft are the opposite, being unable to maneuver as quickly as missiles but being able to constantly add energy. Some missile evasion tactics exploit this asymmetry. But of course, for every countertactic there is a counter-countertactic. The new innovation in missiles is dual pulse missiles, which have two rocket motors so that after firing, coasting, and approaching the target, the second pulse can fire and give the missile more energy to defeat countermaneuvering. The MBDA Meteor is an example of a missile with a dual pulse motor, except that instead of having two motors, it just has one which can be turned on and off.