16+ mile training run

I knocked out a 16+ mile training run last weekend - my longest run since last summer. I hadn't planned on running so far since I have a Half Marathon scheduled 8 short days later.
I showed up to my Saturday morning group training run planning on 8-10, maybe 12 miles depending on who else was putting in a longer run. Jim, who I refer to as the "Energizer Bunny" 'cause he just keeps going and going..., annouced he was planning on a 16-mile route so I thought I'd just go ahead and join him. I was thinking it was pretty cool that I'm in good enough shape that I could attempt a 16-mile run on a whim. We had a pretty good training run. I was able to keep up with Jim for about 14-15 miles and then just run out of gas. I had to slow down significantly for the last mile or two. I expect that this 16+ mile run was a little too much considering I'm racing the following weekend, but it's a good investment looking forward to the full Marathon in August...