Ski in NC said:
You would fly faster for a while, then your motor would break. Then you would look for somewhere to land without power.
Actually, you would climb, unless the pilot trimmed for greater airspeed. If you had a prop control, you might well find nothing at all happening (since RPM would not "run away" until pitch stop limits are reached - and that is not likely to happen with a "runaway" unless there was a LOT of fuel being provided...and keep in mind it would be coming from a less-than-ideal source without atomisation).
Then, from 10k feet, IF you had an actual engine failure (which, if it was a crankcase oil issue, would eventually happen) depending on what you are flying, you would have 300 to 500 sq. mi. to choose best landing place. With the requirement of ongoing maintenance and inspection, the likelihood of a diesel runaway I would suspect is about a thousand times less than an inflight failure of a gasser recip - and THAT is quite a rare occurance in cruise.