After todays performance I'm convinced that's what I got wrong in the last 2+weeks, the recovery. I read this on Joe Friels blog:
The real key for aging endurance athletes is frequent recovery time. All athletes need down time on a regular basis. Older athletes simply need it more often than their younger counterparts. I’ve found that most of the 50-and-older athletes I’ve coached over the years need two to three days of easy training following a highly stressful workout. A young athlete may do two of these sessions back to back and then require only one or two days to fully recover. But not the older athlete. How great the stress is that triggers this long recovery block is an individual matter. Every athlete should have a good idea of what different types of workouts demand in the way of recovery.
In the same manner, all athletes need extended recovery periods every few weeks during periods of heavy training. Younger athletes can go perhaps three to five weeks before needing to take a break for three to five days. For the 50-plus athlete it is seldom more than two weeks before down time is necessary in order to prevent overtraining. Extending the period of heavy training beyond this or skipping these rest blocks is likely to result in unrelenting fatigue and greatly reduced performance.