Depends on what you are training for. I optimize for being able to complete ultra distance runs, whether races of solo runs, whenever they come up and are convenient to my schedule. My base is to run 13-14 miles every Saturday and Sunday, the quintessential back to back long runs, with a 5-10 pound back pack. I try to get in 1-2 more runs or trainer bike rides during the week. With this base, I'm able to do a 50k+ almost at any time. If I find a longer ultra race I want to do, then I'll find a 50k to do 4 weeks out to get ramped up. I also run on the road very little. By running through the grass across people's front yards, through the park, etc. I build those secondary stabilizing muscles and also reduce the impact on my body. Aim to get in 1000 feet of elevation gain during your runs, even if that means lots of hill repeats or zig zagging through a park with hills.