Lower back pain after shoe switch (Read 22 times)

Dear runners,

I have been running for a while now (about 5 months). I recently bought a pair of Nike Pegasus 38s for my long runs. I used to run in a pair of Kalenjis (the Decathlon brand. These ones have a lot less cushioning and a lower stack height than the Pegs). I have not broken the new pair in as much (I have barely run 12 miles in them). I have noticed that when I run in the Pegs at my long run pace, there is a dull aching pain that sets in my lower back on the left side (I am guessing the sacroiliac joint)  It is relieved on stopping the run and taking a 30 second walk. It sets in at about 30 minutes, and seems to worsen with time. Its kind of odd how it is mildly relieved when I up the pace a bit.

This worries me, as I invested quite a bit in this new pair (I am a student, and this pair was a little heavy on my purse) and I really don't want them to go waste. Couple of questions:

1. Will it get better with time, ie when I get used to the shoes?

2. Do I need to raise the pace of my long runs?

3. Are there any mods that I need to do to my run technique or the shoe itself?

In case it's relevant, here are some details about me.

21 year old male, 103kg, 5' 11", long run pace 7:50/km, avg. 5k race pace 6:35/km, avg. Weekly mileage 19-22 miles, not doing speed work except a few strides here and there and an occasional tempo run

Thanks in advance!