I found lenzlaw's links to be very helpful in improving my form.
Since I've started focusing on improving my form in training, I've seen a huge difference. Not so much in my speed, but in my consistency and my ability to run further without hitting the "wall". My pace is MUCH more consistent now and I can speed up or slow down more effectively, based on my breathing and how much I'm breaking form. And I can definitely run further without feeling pain in the areas I was clearly stressing before focusing better on form.
The things I'm currently focusing on that have made a huge differerence:
1. Keep your spine and head straight. Don't lean back (had the same problem you did!), as you'll slow yourself down unnecessarily. But also don't lean forward. You'll cut off your air that way. "Lean" forward at your ANKLES, not your waist. Big, big difference...
2. Breathing. Breath into your belly, not the top of your lungs. For me, 2 in 2 out works well. 2 in means one good breath, then a shorter "finishing" breath to really fill the lungs. 2 out is the same, only even more so. Getting the CO2 out makes a big difference in my ability to maintain a consistent pace.
3. Most importantly: RELAX. I was tensing my calves, trying to "pick them up" on the back swing. By relaxing my lower legs in particular and focusing on using my larger upper leg muscles to drive my legs forward, while letting my legs fly backward without resistance, I've reduced a lot of pain and fatigue I'd been experiencing.
4. Use your upper body! It's really half the battle. Keep your arms at around 90 degrees and pump primarily forware and counter to your leg motion, but never across your body. Makes a big difference.
I'm still having a hard time keeping all 4 of these things in mind at any given moment, but when I find I'm struggling, if I check each one and correct it if I'm out of sync, I find I quickly get back into more comfortable and consistent stride that works for me.
Good luck!