How much rest do you think? 2 days 3 days?
It depends on how you have been training. If you have just been rocking for months then take some serious time off. 2 off bike and easy riding for next 4 days after that. You should be totally recovered and ready to go again.
If you have been following a 3/1 or 7/1 training plan (3/7 weeks on, 1 week easy) then where were you in your training cycle? It might be that you were just dehydrated, sleep deprived, unmotivated or simply the flow/intensity of the race.
I think the best way to see if you are over training is to keep track of your resting heart rate in the morning before you get out bed, then to track your HR during a race. There are too many factors that could influence your low HR during the race. You should have close to the same HR every morning, as you train more and getting in better shape that will start to get lower...months here not days or weeks. If you see your resting HR going up and you are still training hard then you are possibly over training. Talking like 10 bpm here not 1 or 2.
Don't train more and expect to get faster, train harder and smarter, you get faster by recovering. Question, do you feel faster after a 100 mile ride or before? Duh, before, after sometimes you can hardly use stairs. You ride to hurt your body and then let it recover, when your body adapts to the stress you put on it training, it will be a little stronger then it was before. If you train too much you are not allowing your body to recover to that point where it is over compensating making you faster, you are only futilely hurting your body.