I start a new job tomorrow, which is based just under a mile from home. Ideally I'll walk/cycle most of the time, but it's along a national speed limit road without pavements, so with that and the weather I'm likely to be easily convinced into the car. How bad for the car is it to drive such a short distance and switch off? Is it worth taking a detour to warm up properly, how long should it be ideally? I'm intending to keep the car for at least several years, so long term effects do matter. It's an Impreza 1.6GL if that makes any diference.

 

Short journeys aren't good for any car, but if you do use it I'd let it warm up & if possible take for at least a couple of miles before turning it off.
One off short trips wont do any real damage, but if your intending to do this regularly it's best to sort out a longer route.
The problem is that it takes a while for the oil to get around the engine. You could try using Castrol Magnatec oil, which , in theory, sticks to the engine so there's less wear on start up.
I'm sure others (with more Subaru specific knowledge) will have more input on this though.

 

There is no reason/advantage taking a longer route because it would still involve the smaller route within it (the beginning). The only way around this is to start the car 1st and leave it running for a few minutes before you drive it. Get an alarm which starts the car remotely, this way you can get out of bed, get ready, start the car, have breakfast, go to work. As the previous post says magnatec is probably a good oil to use in this situation.