Houston, we have a problem!

For about a year I've been doing the school run running, which means running on Sunday, Monday, Tuesday, Wednesday and Friday. On Thursday I ride my bike into town which is about 12 miles. So I have only one rest day, which is Saturday. Sundays I do my long runs.

And then one morning, about four weeks ago, I woke up and I felt totally exhausted. And when I tried to run, my heart monitor told me my heart was moving way faster than my feet. I had to start walking to stay 'in the zone'. So now I do the school run walking, while wearing my running clothes. I use my heart monitor to make sure I stay in the zone so at least I still get a work out. But it feels like a failure and I've been wondering what has happened?

Haven't I been giving my body enough time to recuperate between runs?
Is it simply that I'm growing older?
Or is my body suffering from Winter blues?

Any insights? I don't want to lose my identity as a runner!