Sounds like rest, form and
Sounds like rest, form and speed. Using new muscles or the same muscles in a new way is always more taxing. Also, I can run 9 miles or so at an avge of about a -10m/m pace, but if I dip into the 8s and hold low 9s, I'm usually spent by about mile 3, have to slow down and am more sore the next day.
It's hard to say without seeing you run, but if your hams were toast and your achillies hurt, it makes me think your clip was a bit fast, not enough rest since your last run, you're over striding or up on your tippy toes and not letting your heal come down. If you haven't already, read everything you can get your hands on about form. Search 1-2-3 landing if you haven't already read "Barefoot Running Step by Step" by Ken Bob Saxton.
Also, if you've been running in shoes with a heel for a long time, it will take a while for your achillies to stretch back out. You've been basically walking around and running "down hill" if you think about it. Low in the front, high in the back. That can cause the tendon to truncate. It'll stretch back out, but not over-night.
Just listen to your body...you'll get there!
-Jonny