I was fortunate enough to be shown around a U-2 at a USAF base by a chum who'd been a U-2 pilot but was now involved in another activity.
The following day a couple of us joined him at the site for the departure. The pilot was being pre-oxygenated in his suit in one of those huge American armchairs whilst the jet was pre-flighted and readied by the support crew. Then he was transferred to an astronaut-style portable life support device and we followed him out to the aircraft in the chase car - he was quickly strapped in and waited for the green signal, before taxying the aircraft to the threshold. We did a quick FOD check along the runway, then turned back to position alongside the cockpit. My colleague gave him a thumbs up, whereupon the engine was slammed to max thrust and off he went; we raced down the runway behind him and collected the pogos, then drove back to the facility.
From leaving the pre-oxygenation chair to take-off took about 5 min at most - and everything was done without radio. A very slick performance which had obviously been practised many times before.
By the time we parked up for breakfast, the U-2 was already in the stratosphere!
Having watched another U-2 landing at another location, it's obvious that it isn't easy!