Rust. corrosion and developed 'acids' as byproducts of combustion are the killers on the combustion-side of any engine.
Short runs dont allow the engine to 'heat soak' long enough to drive or 'distill off' the WATER that reenters the crankcase oil after cool-down. When you shut down, the 'equilibrium' of the oil dictates, that the oil WILL become 'saturated' with water (from water vapor in the atmosphere) ... and if you dont raise the oil temp. sufficiently to essentially 'boil off' the water, that water 'activates' those combustion byproduct compounds which then become 'acids'.
So, RUN the engine at the rpm and with LOAD that quickly raises the operating temp, and keeps that operating temp. high, as long as possible ... nutherwords: RUN the engine as 'hard' and as long as possible.
For 'iron' engines this will also more benefit the 'wet side' of the engine as the 'real killer' of marine engines is internal rust of the cooling passages including 'the block'. Various 'irons' when 'blued' (a formation of BLACK rust or ferrous oxide) is developed. Ferrous oxide forms when iron is boiled in water; only to slowly change back to destructive 'red' ferric oxide over time when the wet iron is at ambient temperatures. Takes an hour or more at 'boiling temperatures' to covert the destructive (ferric) 'red' rust back to protective (ferrous) 'black' rust.
Rx: Get the engine temperature up to 'normal' operation temperature and ~75% of max. rpm as quickly as possible by applying a 'load'; and, RUN the engine at the normal (180° typical) at ~75% rpm as long as possible - typical minimum is 'about an hour', longer is better.
