>Running 101>running form or stride?
One day at a time
Go back to the old stride.
The changes that you describe were causing you to overstride which is just the opposite of what you should be working toward. Overstriding forces your foot plant out in front of your center of gravity which slows your pace and puts a lot more stress on your legs. It probably felt better on your run because you were using muscles that you don't normally use that weren't fatigued.
If you want to learn more about what constitutes proper form I would suggest the book Programmed to Run by Thomas Miller. He does a great job of explaining what good form is and offers numerous exercises and drills to help you achieve it. You'll likely find that even your faster rate is still slower than ideal.