i've been doing trail marathons and 50Ks for a 3.5 years and did my first 50 miler a year ago; i've since completed (barely) a 70 miler and two other 50 milers. i don't aspire to run a 100 miler, since running in the dark puts a real strain on my eyes and head. i have had the greatest success, and achieved the most even pacing, when approaching them as a long (long) easy run and then pick up the effort with about 10 miles to go.
as an older (48) female, i typically run 70-85 mpw including an up-tempo day (shorter road or trail race, long-hill reps (5x15-min hill), or tempo run), and should do track work but mostly don't get around to it. my long runs vary from a low of 2.5 hrs to 50K trail race, and sometimes use back-to-back 15+ milers as my long stuff. i run only singles, both due to time constraints and because i enjoy being able to cover more terrain in a given outing. while doing this kind of training, i've also seen my times drop in shorter distances, not only on trails but also lowering my 10K road PR.
last year i ran a marathon-distance-or-longer race every three weeks (with one interval being only two weeks), and found myself spending too much time recovering and/or tapering. this year i'm spacing out my long efforts more, and being sure to put at least four weeks, and usually longer, between them in order to train more consistently.
i have one or two more 50 milers on tap this year (white river and maybe IT 50 (see below), having completed two already (bull run run and sulphur springs)), plus the pikes peak marathon.
i'll put in a plug for a new 50-mile/100-mile trail race to be run in the finger lakes region of new york this september: the iroquois trails 100. it's run primarily on the beautiful, wooded (and sometimes rooty) singletrack of the finger lakes trail, with a detour up and over (and over and back) a ski hill. this area of the FLT has very few rocks, several long grades, and no significant creek crossings. the race has a total of ~18K' of climb for the 100 and will be amply supported.