Sorry mate, but unless you are on full throttle acceleration (and probably even then), aluminium will conduct heat quickly enough to detect this. Also you are not correct in respect of the PCM (ECU) measuring the water temperature to determine this, it actually has a sensor directly at the back of the head, and not in the water, so totally independent As far as your hose cracking, then this has nothing to do with how many k's your car has travelled, but depends on age, mechanical damage, general condition of cooling system etc. Like drive belts, hoses are worth checking every year or so, and replace if any signs of damage or ageing are found.