Strangely disappointed

Don't be disappointed in yourself. There are lots of factors that go into how tired you'll be after a particular workout. Instead of being bummed that you weren't exhausted, be happy that you'll have a bit more energy for your workout today.

The next time you run, you'll know it's okay to push yourself harder and farther. Besides, I think it's better to realize that you didn't go far enough than to realize you went too far -- you know, undertraining versus overtraining.

BTW, congrats on your new milestone!

Good job. Don't be disappointed! You should only train about 10% of your weekly milage at a speedy pace anyway. The bulk of your miles should be done at a comfortable pace. You're supposed to feel like you have gas left in the tank after most of your runs. Next time you go out for 5, do the first 2.5 at comfortable pace, then gradually increase the pace until the last half mile is at your race pace. This is called a tempo run and should leave you tanked.

I'm with Bigolbro on the steady pace. I spent the last 6 months always running well within my limits eg slow gentle pace but longer distances, and finished most runs feeling I had plenty left in the tank. The result has been a MASSIVE improvement in my overall endurance, no injuries, fast recovery and plenty of energy to devote to the other 2 sports. It was really tough to avoid the faster pace work but it has paid off big time. Many runners say you should spend at least 1 year just building your base endurance in low intensity distance runs (60-70% max HR) before you do any speed runs. My experience is that this is a VERY good idea, so don't give yourself a hard time about it.