Re: Is there something as to much endurance work?
I agree with Robert above me here. If you were only training for running or biking for example, 6 days a week is not that uncommon, but the combination of the two is another story.
It's difficult to train both optimally together - Crossfit hits both and for general fitness it's enough with the 2 days on and 1 day off schedule. That works out to 4 days one week, 5 days the next approximately.
If you want to add more endurance I do recommend intervals - not because they have magical power - but because the break in between does make recovery easier. You can improve your aerobic endurance with longer ones of 3-5 minutes, and your anaerobic endurance with 30-60 second bouts. 2 times per week is enough if starting. You don't want to go all out - but keep it pretty intense - like 80% on the longer intervals and maybe 90% on the shorter. How long you do it for is up to you. Certainly build up. Listen to your body. Give it a go for 6 weeks and observe how it impacts your other training, and decide what needs to be cut out if anything.