I've decided to be very sad and record all my journeys this year, I was wondering how I work out what the mialage is, and if I get say the Stockport only train from Manc Pic to Euston, how I can know which direction it's been without the wonders of GPS or looking out the window...
The only answer is you have to pay attention! Whilst many trains in this country will only go via their booked route, there are several instances where you can get caught out; for example going via Northampton rather than Weedon and via the West Midlands rather than the Trent Valley. Though you would have to be well dossed out not to notice the latter

leepy2:
Being a dinosaur, I still do my mileages the hard way using a "Mile by Mile" book (David Maxey / Peter Watts publishing, now out of print) in conjunction with Quail / Sectional Appendices for up to date bits and fine details. I have made my own "conversion table" to decimalize the miles and chains and write all my moves out on a spread sheet so it is easy to add up.
Foreign stuff is easy being metric, there are plenty of books and atlases to get distances from - if you are in a country where there are no known books of distances, then you will have to stay awake and watch the km posts. I always convert my km distances to (decimal) miles, it provides an interesting comparison.
One last tip, keep up to date with your moves as it is an absolute pain to try and catch up, speaking from experience here as I am currently trying to catch up on over a years worth.