Well, have you ever taken a 5K out in 5:45? If so, what happened next? In other words, just do it! When faced with a similar situation many years ago, I went to the track, stopwatch in hand and just ran the damn thing, timing each lap along the way. It will either go right or it will go wrong. Either way, you will know if you have the guts or not and at what point your 'guts' run out. If you start to die at 1.5 miles, then set up a run on the track covering the first two miles, again timing the laps. When you do that, then try 2.5 miles, etc. Get 'er done!