I made a really ugly but functional spreadsheet in Excel. Then my wife saw it & freaked out (she's a former presentation graphics professional) so she designed a really attractive spreadsheet in Excel; that's what I use now.

I just log miles per day, & it gives me a weekly total & a running YTD total. I don't bother indicating anything about the ride (terrain, weather, elevation gain, etc) but I do occasionally include notes to remind myself why a particular weeks mileage was higher or lower than average.

I'm about to modify it so that it'll give me a "Check Chain" warning every 1500 miles or so.