People often advocate entering a long runway at half length in order to expedite departure. My own view is that it's foolish to throw away the opportunity to land back on in the event of an engine failure after takeoff, so I always start as far back on the runway as I can as it gives me an extra safety margin.
Reminds me of my instructor telling me about the three most useless things in aviation:
- the runway behind you
- the air above you
- the fuel in the bowser.