> and take days off if the signs of overstraining appear
Your training plan should include days off irrespective. And also easy "rest" weeks. When training you essentially do damage (wear and tear in the muscles etc.), and when resting after the body repairs things better than they were before. If you never rest properly, the body is fighting a losing battle and you'll never improve as much as you are trying to, if at all. This is why marathon training plans and similar tend to be in 4-ish week cycles with an easy week every cycle and explicit rest days even in the hardest of weeks. Even ultra-marathon training where you might have back-to-back long runs will have rest days each week and whole rest weeks within the program.
Caveat: I'm not expert so consider my thoughts anecdotal.
Though I'm fairly confident I'm not talking complete rubbish as I've run a few road marathons (best time so far is just shy of 4 hours, I'm not sure I'll try for any faster than that), plenty of road halfs, a number of trail runs of 13 miles or more, one trail "ultra" (30-ana-bit miles so only 4 above base marathon distance, but bugger me there were hills/steps/mud/other!), and a long weekend trail challenge (69 miles over four days), and I'm training for another trail marathon (~28.5 miles this one) next month and another trail challenge weekend (100+ miles over four days this time) later in the year.
I typically "train" every day, meaning I run six days a week and ride my bike on the seventh. I do, however, have rest days; it's just that my rest days are shorter distances at a lower heart rate. My Tuesdays and Thursdays are rest days, so I run 8.9 miles @ 130 beats-per-minute heart rate.
I absolutely, do not recommend my training plan for others; it's taken me more than ten years to get where I am, but as an example of what can be done (at least with my body), you can see how I recently trained for the Bataan Memorial Death March[0] and also see a subset of my official ultra results at UltraSignup[1] as well as my self-reported marathon results at Marathon Maniacs[2].
FWIW, I think my sleep schedule and diet have contributed significantly to the gains I've made over the years, but I'm also well aware of survivor bias. Neither of my parents were athletic though and I absolutely hated running as a kid (although I really enjoyed ultimate frisbee in high school).
I suspect that ultra endurance is somewhat different from the other sports. It seems reasonable that conditioning requires loading mileage like crazy, even compared to marathon or triathlon. Maybe microcycles (1 week) are unnecessary.
Modern preiodization was scientifically discovered by Russians in the 60s and it seems to work in every sport when correctly applied. Do you have have mesocycles (2-5 weeks) or macrocycles (6-12 month) in your training plans?
Your training plan should include days off irrespective. And also easy "rest" weeks. When training you essentially do damage (wear and tear in the muscles etc.), and when resting after the body repairs things better than they were before. If you never rest properly, the body is fighting a losing battle and you'll never improve as much as you are trying to, if at all. This is why marathon training plans and similar tend to be in 4-ish week cycles with an easy week every cycle and explicit rest days even in the hardest of weeks. Even ultra-marathon training where you might have back-to-back long runs will have rest days each week and whole rest weeks within the program.
Caveat: I'm not expert so consider my thoughts anecdotal. Though I'm fairly confident I'm not talking complete rubbish as I've run a few road marathons (best time so far is just shy of 4 hours, I'm not sure I'll try for any faster than that), plenty of road halfs, a number of trail runs of 13 miles or more, one trail "ultra" (30-ana-bit miles so only 4 above base marathon distance, but bugger me there were hills/steps/mud/other!), and a long weekend trail challenge (69 miles over four days), and I'm training for another trail marathon (~28.5 miles this one) next month and another trail challenge weekend (100+ miles over four days this time) later in the year.