Bad Running Day

I ran ten miles today. I was scheduled to do a tempo run, which I had planned to do as follows: two mile warmup; one mile at goal marathon pace; four miles at tempo pace; one mile at goal marathon pace, and an easy two miles home. As it turned out I couldn't even maintain the pace for the first marathon pace mile. I was running into a pretty stiff wind, so I just tried to go by effort; and then when I rounded the point, I increased the effort to try to reach tempo pace, but even with the wind at my back, I was still running slower than goal marathon pace. I just didn't have the energy to run hard. I ran this same workout a couple of weeks ago. It was a tough workout, but I finished it. I have to assume that my problems today were the lingering effects of giving blood last week. I decided to just get my mileage in this week and not worry about pace. I had to push my marathon back a week, so I will use this week for mileage only and get back on my schedule next week. Hopefully the extra week of easy runs will get me back to where I was.