I'll be riding in my first century on August 15. Over the past four months I've built my mileage up to 150 miles per week. (25 miles on M/T/W/F before work, and 50 miles on Sat.) With less than 4 weeks out, I'm worried that I'm feeling the effects of over-training.... Extremely tired all the time, diminishing motivation and slower times. I should be close to peaking, but instead I'm fading.
I'm not sure if I should continue to push through this period, and keep my mileage up, or back off until my strength returns. My diet is about 70% carbs, and I'm getting 7 to 8 hours of sleep at night. I'm 50, and in reasonably good shape. Have riden for almost three years.
Pretty much what you already know, you need to rest. Take a week or two of reduced mileage. If you feel like it you may put a little intensity (sprint) into it, but never go home feeling tired. You're body will thank you for it and reward you with new found strength. Don't worry, you will not go out of shape by taking couple of weeks of active rest on the bike. You'll be better off than trying to tough it out for the next 4 weeks. Also, don't skip on your meals after rides. They're crucial for good recovery and adaptation.
I don't train for centuries per se, I just train for races however long they may end up being. Here's a little post that I wrote couple of months ago about my first 100+-miler:
personally I like training one week harder on myself then alternate one week of slower paced or easier riding to recover then back to a high paced challenging ride for a week.
