I can't sing RailMiles' praises enough, it's simply an amazing tool and I don't know how I got by without it. I still try to keep my paper records as accurate as possible, if for no other reason than to have a look back one day and to have a non-electronic backup, but RailMiles is worth every penny of the subscription fee. My non-Premium account was something like £6.60 for two years, an absolute bargain.
If memory serves correctly, Tom does a backup at least once a day so all the data is always secure from crashes and the like. Whilst spreadsheets and all that are handy to have, I prefer RailMiles for all my mileage needs these days. Except for bits like freight-only lines or preserved railways, then I get out the Quail and Preserved Diesels for the mileage gen.
If you want the mileage that a particular train (or overall journey) takes before you take it, then look it up on any WebTIS-powered booking engine (e.g. East Coast, London Midland, Southern etc) and hover over the 'carbon emissions' link; the URL will contain the mileage (rounded down).