On getting slower

This morning i ran the George Washington Birthday Classic 10k here in Alexandria. The first race since September when i ran the chicago half marathon (pace: 8.12). Well the GWBC was the slowest pace i've ever run in my life for a race under 10 miles. Unofficially crossed in 46.45 (pace: 7.32). Why is running so difficult to maintain. you fall out of shape so quickly it's ridiculous. Makes me just want to lift huge amounts of weight and try to get bigger, that seems easier to maintain and requires less effort and sweat.