> and take days off if the signs of overstraining appearYour 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.