Just Keep Going

Only 3 weeks left until the 5k run.
I am trying to visualize myself in it, I can see myself running across the finish line.  It's the start and middle that I can't see.
I'm having a hard go with the running.  I start out okay, but if my mind starts to wander I forget what I'm doing and my feet stop.  I try to tell myself to run through the hard bits and I slowly do that but again my feet just stop.  So with 3 weeks left I need to move past the hard bit and concentrate on the running and just keep going.
AHHHH