There are not a lot of possibilities. Interval training is about a reference time. While for sure “10 x 2minutes hard with 45sec recovery” is interval training and can be done on any terrain even while running continuously, it still leaves a couple of unturned stones behind. At what elevation, on what type of terrain with what altitude gain and loss, wearing what clothing, on what slope angle, what type of technicality and humidity in the air and on the ground, on what distance and more. If you really wanted to improve, it is impossible to train on a different course each time you go out. How do you know, that you got better ? This is the goal, isn’t it ? On the track, it is easy. Always 400m, always flat. You really know if the wind was blowing or not. Always in singlets and shorts, winter time might be in tights and in a second layer. The environmental factors are really non-important comparing to trail. This is why I recommend even for ultra runners to get often onto a track and to keep a controlled interval training program in their magic hat !
Option 1: Fixed course
There are however runners, who do not have access to a running track. They still should be able to train outdoors professionally with a simple timex watch.
I highly recommend to find multiple reference courses. Flat wide open tracks, rolling hills, steep climbs, downhills. You could use a GPS to measure out distance, but it is unnecessary. Just use a tree, a barrier, a gate, a trail-sign or any very recognisable landmark, to put a timeframe in-between two limits. It can be even half road and half trail.
Use track reference times to set up your distances. These are high intensity times. 200 / 400 / 800 / 1000 / 1600 times and more. Basically you can find trail sections what last for 25/35sec + 60/90 sec + 2/3min + 3/4min …
Practice on them like it was a track. Do not try running a PR every time or decrease your times regularly. Just do the same workout over and over again. Then after the race, if you felt like a better runner, like you stepped on a higher homeostatic level, you might drop a second off your times.
Be smart about setting up your different distance tracks. The best is if they are close to each other. Maybe you can even find some circular ones, what increases your possibilities. This is crucial, in case you wanted to mix intervals. Let’s say 4 x 35sec + 4 x 1:20 + 4 x 35 sec + 10 x 10 sec sprints. Also you can do ladder type sessions of increasing and then decreasing distances.
Of course varying your training and playing with recovery times and efforts are crucial. Running a 5 x 35sec all out with 3 minutes recovery cannot be as fast as 20 x 35sec with 35sec recovery.
One thing to remember and to really dial in. Effort. If you never ran a road race, never ran on a running track or never really did like an all out 8 to 10km non-technical trail race, you might be totally unfamiliar the efforts necessary to complete these preset sections. The risk of over evaluating yourself has no consequences outside of dropping your times and slowing down. However if you really don’t understand your body or the intensity and you train too easily for your needs and fitness level for 5 months, that is kind of lost time. It might even result in detraining or simple maintenance.
I see very little tuned-in runners. It is mostly overtrain or under-train. Mostly these two assets are caused by lifestyle and run technique factors, not necessarily by training itself. Living badly do not let them train hard enough or on the contrary, do not let them recover.
Coming back month after month, year after year to the same trail sections, will give you an idea about your long term development and about your needs for the future.
I have one advice though. Do not run only technical ! You must include sections, where you can open up your stride, where you can really focus only on running and disregard the terrain. Very often, even the most technical races like Trofeo Kima starts and finishes with road. If at the end of a 50km mountain race you cannot cover the last 3 km road downhill at 3:35/km just at 4:15/km pace, you might even loose 4-5-10 places ! That is a whopping 40seconds loss per kilometer, resulting in 2minutes total. In case of shorter races like 10km, it is even more significant. Starting out 3km @ 3:25/km and finishing the final km at like 3:15/km like in one of my previous races. Despite that you were the fastest on the real tough terrain, if you lost 3-4minutes on the concrete sections, road runners and triathletes will dominate you !
We can find them everywhere, all over the world ! We can use them as reference to training of course. However there is a very useful function, especially if you can get out onto the race course.
Comparing your training times against fellow runners on certain segments of the course. You can go out and run like a 10 km section of a trail race and see what you can do on 3 segments of uphills, downhills and flats, each of them measuring 1 to 2kms.
This will already give you an idea about finishing time, but also about the fitness you need to further develop. That might be downhill running or advancing on rough terrain, increasing hand, eye, feet coordination and balance plus core strength.
You have to know yourself. Some sections might genetically not suit you, so being confident in them is more important than speed. However other parts of the trail will be your forte and you can win all the time back that you lost !
Even for a trail ultra, there are tons of strava segments. If you can keep up with them even during the end of a 3,5 hour long run, then you can be more sure about your future success and that your preparation is going well.
You can also check out how the runners of the previous edition perform on road and trail segments closer to you. If you can match up or beat their training times, well, you know that during the race you can hold on to them.
I do not really care about breaking a strava segment and getting a KOM. Don’t get too immersed in ego-jerking of collecting fast times. It will result in injuries and burn out. I recommend mostly to train properly at your own pace, convenience and time. When the training-plan calls for a 5 or 10km tempo, use a course and a GPS watch to see where you are. Check to segment times, come back in a month.