Eighteen miles. I'm pretty sure that is my long distance sweet spot. Eighteen miles (or thereabouts) seems to be the long run that I can do relatively "easily" with few repercussions afterward. Maybe it is because eighteen miles takes about three hours to run...maybe three hours (or so) is my comfortable run time limit? Let's examine the evidence....
I have done the Birch Bay 30K three times in spring marathon training...each time it has been a happy experience. In 2010 and 2012 my pace foreshadowed my upcoming spring marathon pace (it did not in 2011 because the Boston Marathon course was much harder than the Birch Bay course).
I have a history of 17-19 mile training runs where I have gone out the next day and done a sprightly 6-7 (or more) mile run at a reasonable, sub-10 pace.
Last summer and this spring I did 18-milers on a Friday and followed up on Saturday with a fast 8K-ish race. In July 2011 I PR'd in an 8K at 39:32, and in April I ran the 5+ mile Tulip Run in 42 minutes flat.
I've started noticing that while I am expectedly tired and sore in the afternoon after an 18-miler, I don't usually stay awake that night with achy legs, and seem to feel fine the following day (to the best of my recollection, anyway).
The most recent example was this weekend. I had one of my longest running streaks over the weekend (anything more than two consecutive days is a streak for me). On Friday I ran 7-ish miles at moderate effort. On Saturday I ran 7-ish miles which included a 24:30 5K race (second in AG, it's a small race!). On Sunday I ran 19 miles (oops, had planned on 17-18). Then on Monday I was back out for another 7.5+ miles at moderate effort (sub-10). Today is a non-running day but my legs hardly feel like they need to rest at all! This is kind of a big deal to me because my ankle/achilles/heel has been bothering me lately. After the long run I soaked my foot in a bucket of ice water for 15-20 minutes (allowing it to thaw every few minutes), and perhaps that helped. I really need to be more regular with icing.
The other thing I have noticed is that in the marathons my legs really start to feel the ache after 18 miles. In Boston the downhills really did their thing on my quads over the first three hours or so, and although the Newton hills didn't bother me, the final five miles downhill to the finish were agony. Last fall in Portland, even though the race was great overall, I really started to notice my quads after mile 18.
Coincidence? I think not. Some marathon training plans don't recommend doing a long run of more than three hours. (I'm not going to cite to anything because I'm just drawing on memory here. Highly scholarly research, I know.) I don't follow that rule because as a moderately slow runner, that would really limit me to 18 miles, and I do want to go further, on occasion.
Liking the 18-milers isn't going to keep me from the 20+ mile runs, of course. (And I did notice, in my review of Garmin stats over the last year, that in August 2011 I did a 20-miler at sub-10 pace, then ran 7 miles at 9:20 pace the next day.) In fact, my ultra training has me doing some looong runs this summer. I'm hoping that the extra distance (and hours) doesn't beat me up too much.
Maybe I can make my eighteen mile sweet spot stretch to 20, or 22...we shall see.