What did the pilots of the gliders do after their units landed?
Glider pilots were trained to fly gliders, but also to fight as infantry (both US and British pilots had to qualify to some extent with all infantry weapons) alongside the troops they had landed for as long as was necessary (a few days, even as long as a week or more) until they could be evacuated to the rear. Many glider pilots were killed. Unlike normal infantry, glider pilots were more expensive (and took more time) to train, so they needed to be protected and reused if necessary. In the case of Normandy, the order to return to the beaches (for British pilots) for evacuation was given on D+2. During Operation Market Garden, glider pilots fought for the whole period of the operation.
Sources:
They Flew into Battle on Silent Wings: World War II Glider Pilots of the US Army Air Force, by Maj. Leon B Spencer, USAF (ret.)
http://www.ww2gp.org/pdf/TheyFlew_into_BattleSilent_on_Wings.pdf
United Kingdom- Glider Pilot Regiment
Max Hastings, in Armageddon, has a much less charitable view of the glider pilots and claimed most actually just melted away into the countryside without fighting.