Question for diesel gurus (strange failure 6.9idi)

Haven’t started my truck in about a week.
It was 50* out so I didn’t use the block heater.

(it’s a ‘87 F350 dually. 6.9 idi/t19 4 speed)

Started right up after a few cranks, then I had it idling for 5ish minutes. I do this a lot with this truck because it doesn’t have tags yet.
Had it warmed up and went into low idle like it should.

Anyway, it randomly started revving up without me touching the pedal, and I immediately killed it with the ignition. Only did this for about 1-2 seconds before I cut it off.

Looked over everything, no leaks, didn’t blow up, nothing got sucked in the intake.

waited about 5 minutes and went to start it again, seemed like it was cranking faster than normal so I stopped.

What the hell happened and how do I fix it?
It’s a low mileage truck, but I suspect it might be injection pump failure or lift pump failure?