Resuming speed training

I was a bit uncertain concerning my knee's after Yesterdays workout. I have been thinking a great deal about what the real cause for the pain in my knee's could be. Possible causes are to much strain, insufficient technique, wrong shoes, low strength in feet. I think nr 2 is connected to nr 3. Today I used a pair of competition shoes(weight: 193 kg), Mizuno Wave Ronin. I didn't feel any pain or discomfort in my knees at all. I could clearly sense that I tended to use my forefoot to a greater extent and a more evenly distributed pressure when landing on the foot. In the time to come I'll use these shoes only and check out if that's the reason for my knee-problems.
Well then, about todays workout. I did four, short intervalls. The speed was there like before. In order to develop I need to increase the length of my intervalls. I'm running 435 m(0,270 miles) now. Next week I'll increase the length to 600 m(0,37 miles).

Technicals:
-Distance: 5,36 km(3,33 miles)
-Time: 32m 38s
-Average speed: 6,09 min/km(9,8 min/mile)
-Elevation gained: 22,9 m(75 ft)
-Calories:  372 kcal
-Average heart rate(max): Not monitored.

The GARMIN logfile can be found at:
http://connect.garmin.com/activity/41044699