You said your partner runs a few "small ones" a year. You do know that the definition of a marathon is a 26.2 mile race, right? Other races (like 10k's) are just called races. Anyway, the way to train for increasing mileage is to only run 1 long run per week. This is because the benefits of the long training are outweighed by the overtraining effect and need for recovery if you run your highest possible distance more than once a week. So pick the best day of the week for you and designate it your long run day. Start with however much you can run and increase it by 10-15% every other week. Calculate ahead of time where you are at so you know how long it will take you to get to the 17-18 mile range. You should also run 2 other times a week. One of these runs should be something like hill intervals to maintain leg strength. The other run should just be a relaxed, mid-distance run. Never actually run 26.2 until the marathon, just top out in the 17-19 mile range for your long run, and you don't even want to be doing that for too many weeks (maybe 2-3). The week before your marathon run only twice, one long run that't only 10-12 miles and one easy 5-6 miler. If you try to do this training regimen, I will tell you ahead of time that when your long run gets into the 10 mile range it will get very difficult to maintain the discipline and focus to stick with it. Just push through this phase and keep at it, wit a little grit and determination you'll have yourself ready for that marathon. Good luck.