Blah, blah, excuse, blah but I ran back to park and ride from work on Monday (21 miles) and was just cruising. Also ran 50k today (Saturday) again with very even splits and the last lap the fastest. And faster than last week by probably 15 minutes. My other big workout is on Wednesday. That's a track workout - 16 miles with a current goal of 2 hours. My fifth mile was 7:25 but my left knee started hurting on the last lap so I stopped. I'm blaming it on crashing my bike on the way to the track. I turned quickly, my pedal hit the ground and then I hit the ground. My bike is a fixed gear which means I can't coast. I also changed the crankset recently from 160 cm to 170 cm for mechanical reasons - that made me more prone to pedal strikes. Blah blah excuse blah.
I've decided to take iron after each of my big workouts. Iron seems to be my key for avoiding dead legs so I'll try to take it a little more often than once per week.