A few days ago I had a very painful run. My old injury in the lower left leg came back and I had to walk more than 25% of the route.
After the run I immediately ordered new shoes -- my previous ones were from November 2010 and completely worn-out. The shoes arrived yesterday and I went out for a run just now.
What a difference! I felt my leg but not strongly and I ran with only my usual walking breaks. Such a relief.