The other night I went for a 10k run on snow, slush and ice. Didn't drink before and during the run. After running I had a banana and a small bottle of water. Up to this point it was going all right. That was when I started drinking with friends and had several drinks during the night. And as a result came the worst hangover I've ever had that lasts for more than 24 hours. I missed one training session and was feeling absolutely fatigue the day after.
If you are a runner you are likely to have low alcohol tolerance. There is a few reasons to that. You probably only drink occasionally, so your body hasn't built up the same alcohol tolerance as someone who drinks regularly. Because you run regularly your metabolism rate is higher and you get the effects of alcohol faster than someone who doesn't exercise regularly. And because you are likely to have less body fat alcohol remains in your body longer.
Alcohol affects your training in different ways. In short term it slows down your recovery process as drinking alcohol can lead to dehydration if you don't drink enough water. It ruins your workout the day after as it increases the risk of unusual heart rhythm so you cannot have an intense training session for a day or two after heavy drinking. In long term it can affect your weight as you get a lot of empty calories from alcohol.
Drinking in moderation doesn't affect your health. But if you are a runner, give it a second thought the next time you head out for a few drinks. I will.
Read more about the matter here.