If you have any long term physical performance goals (as I believe you do), then knowing when to hold them and when to fold them is a very useful thing. My best guess is that erring on the side of caution when feeling run down is a winning strategy, especially if you're talking about a day of training as opposed to a very important race.
Feel better! Half marathon in June for you ....in Oct for me. It's keeping me focused already . There are always more races and more goals.