Runners low

Most of my marathon training has been pretty ok, no real low points, bad runs, lack of motivation or even injury. Until this morning… I was meant to do 15 mins warmup, then run 4k tempo (fast, but not so fast you could taste blood according to the coach), and finish with 10 mins cool down. My hip flexors felt like they weren’t there but had in fact been replaced by pain, and I felt like being sick several times (dry retching on a run is a first for me) – and I was only doing a 6:10 pace (mins/km)! I kept thinking why am I doing this, why even try to run a marathon, what if my legs feel like this on the day, why on earth am I doing outside when I could’ve stayed in bed for another half an hour... Real low point. How does one snap out of it?