Hi Jen - I totally understand the feeling you had after your 8 mile run. I too am training for my first half marathon in May since the birth of my children (ages 8, 6, and 3). I will be 45 next month, and am trying to train very smart by following a 10 week, 3 running days per week plan that was created by the folks over at Runner's World. Apparently many people had PR's using this method, but honestly, I don't really care if I PR - my goal is to train comfortably and run the race in reasonable time (about 2 hours) fairly comfortably without injury.
The plan basically consists of a shorter speedwork run, a longer tempo run and a slightly slower than race pace long run each week, with each run getting a little bit longer as you approach two weeks before the race date, and then tapering off a bit for the last two weeks. They also encourage cross-training on your 'off' days by swimming, biking, walking, or in my case, doing a Cathe video! I will be starting the program next week, as my race date is May 1st.
Any time I went for a long run (8 or more miles), how I felt for the rest of the day depended on so many factors - how much sleep I got the night before, what I ate, how much fluid I drank before or during the run and what the weather was like outside. Personally, I am a cold weather runner, and really struggle in the heat, so I think having enough and the right kind of fluids is crucial to your performance and how you recover. I also think you have to listen to your body and don't try to do to too much more than it's asking. I also found that my body needs a day to recover after a run, so I try not to run two days in a row if I can help it. I have been scheduling a 20-30 minute yoga practice into my workouts (love Jill Miller!!!), and that alone has done wonders for my hips and hamstrings. By gradually upping your mileage, I think you will find that you won't feel as tired as you did after your 8 mile run.
Here is a link to the article
http://www.womens-running.com/article/0,7120,s6-238-244-258-9369-0,00.html. Good luck, and let us know how things are going as your approach your race date!!
Karen