Fatigue Factor

The past two weeks, I've done long runs of 12 and 13 miles. After each run and into the following day, I've been pooped! These runs represent the longest runs I've done....ever. I've gone from running 8-9 miles a week for as long as I can remember, to running 24 miles this week-a significant increase. I'm wondering about the tiredness though and if anyone has experiences or strategies to suggest. My only strategy so far has been to avoid my 2nd daily workout.