My last race had that problem. My second km was run in 4:25, and then I ran a 3:35 3rd km - and that one was mostly uphill. One of my best feeling races I've ever run was without a watch. I ran by feel, and set a pretty decent HM PR. Still, I think that those who are counseling you to take it out a bit slower than your goal pace, and then kick it if you're doing well are on the right track. Nothing kills confidence more than going out too fast and crashing in the second half.