Endurance
On your long runs is where your endurance will show up. Have you put in enough time on your feet in physical miles to survive your distance? There have been a couple of good boundaries, rules of thumb, that when I follow them have served my training well. First is the 3 week cycle of increase for two weeks drop down or back for 1 week, this routine applies to both training based upon time on feet and based upon miles. I learned that with each new training cycle I didn't have to start back at zero miles. I just dropped down to a long run mileage I could manage without strain. When I first started trail running all my training was based upon time on feet. Fleet Feet Nashville had created a trail training group to cover race distances of 10, 13.1 miles, 25k and 50k. My first long run was for an hour and 15 minutes and it felt like forever. From there long runs increased by 15 minutes each week then dropped back by 15 minutes for one week and then would add 15 minutes for two weeks. Long run times looked like this: 1:15, 1:30, 1:45, 1:30, 1:45, 2:00, 1:45, 2:00, 2:15. You can see the pattern that is developing. Then for during the week it was generally every other day for about 45 minutes until the long runs had built up time wise to meet the weekly time, then weekday runs started increasing in time bit by bit. This time on feet has worked well for me for all my distances under 50 miles and I still use it, in part, for my longer distance goals.
As I increased my race and adventure miles beyond the 50k I found I needed to incorporate a different training method. Training for my first 50 mile race with a solely time on feet wasn't working out as it was requiring some very, very long long runs. I didn't fully embrace the back to back long runs at this point because I was worried about being able to complete the distance in one push. Now I have to point out at this point I had a decent set of miles for a base to work off of. My first 50 miler was about 4 years into my running and racing. What this training looked like was to start with the race day and distance and work backwards and factor in total weekly mileage for each week. So 2 or 3 weeks before race day would be the last long run including back to back long runs. For my 50 mile races this ended up being either a back to back long runs with a total mileage of 40 to 50 miles or one long run of around 30 miles. The total miles for that week might be around 50 or 60 miles. With this information I would then work backwards using the idea of up 2, back 1. It's hard on trails to simply say I'm only gonna run 45 miles this week when you only train on trails. So I would break weekly distances into roughly 5 mile increases. When 20 to 30 miles a week was my base miles this wasn't too bad of a plan to follow. It would also allow me to try to get some speed up. I'm not fast, a mid-packer for sure. My first races and adventures over 50 miles my weekly mileage was 70 to 80 miles and the longer the distance I would start do back to back weeks of 70 or 80 miles then drop down 10 to 20 miles on week 3 then building back up. To date when I've trained well I've complete a 100 miler, several 100k's, and a lot of 50 milers.
The big benefit to this up 2 back 1 idea is that if you follow it you'll reduce your chances of being injured. To be honest I don't always to a good job of following the program.