I think weight lifting has been affecting my quality training sessions. Example: Monday I did a short weight session in the gym. My upper body was still a little sore on Tuesday when I started classic intervals on my rollerskis. I lasted two intervals and abandoned because my second interval over the same distance was much slower than the first.
Maybe I was not totally recovered from five and a half hours of biking, running and rollerskiing over the weekend, but I don't think so.
In past years, I found that doing rollerski intervals first, then driving to the gym for a weight session worked best. My muscles were warm and flexible from the interval session, and the 30 minutes of changing shoes and driving to the gym allowed my muscles to recover from the aerobic effort. I'd have a quality weight session without impacting the quality of the interval session.
So why am I doing weights on days between intervals or distance sessions? I don't know... But it's going to stop now. Starting tomorrow, it intervals then a weight session.
The key to training is to make sure you're rested for hard sessions, so you can get your heart rate up.
Lesson: Plan your training schedule so you're rested for your most important sessions.