A marathoner-in-training on my team couldn't understand why she had been really tired and sluggish the day after her long training runs. After some routine blood tests, she found out that she's borderline deficient in both iron and B12, which helps explains her exhaustion.
If you're training for a half marathon or marathon, it's definitely normal to feel tired after long runs. You've expended a lot of energy and put a lot of physical demands on your body. However, if you feel as if you're completely wiped out and sleeping away your entire weekend, here are a couple of ideas to try:
Make sure you're eating properly, especially after your runs. After running, especially a long run, you want to replenish energy as quickly as possible. Muscles are most receptive to rebuilding glycogen (stored glucose) stores within the first 30 minutes after exercise. If you eat soon after your workout, you can minimize muscle stiffness and soreness, and help reduce your fatigue. Find out what to eat after your runs. Also, track what you're eating and make sure you're following a balanced diet. You may not be getting enough iron or protein. If you haven't had a recent physical, make an appointment with your doctor and get a complete blood analysis done.
Avoid overtraining. Running too many miles and not giving yourself any rest days will definitely leave you feeling exhausted most of the time. To avoid overtraining, don't increase your weekly mileage by more than 10%. Try to give yourself periodic "rest weeks" by dropping your mileage by 50% every fourth week. Take at least one rest day a week, by not exercising at all or doing a cross-training activity instead.
Establish good sleep habits. Aim for 7-8 quality hours of sleep a night -- the right amount for most adults. Getting very little sleep during the week and trying to "catch up" on the weekends is not a good idea because it alters your sleep schedule. Your body is forced to adjust to these changes and, as a result, your quality of sleep is poor. Try to establish a more consistent daily sleep schedule.