So you have never done a 50 miler and want to know how to pace it - slowly - you are guaranteed a PB if you finish.
My marathon PB is 3:57 and my 50 mile PB is 12:30 ish. Simply put I run very little of a 50 although ones I have done (6) are all hilly, trail and with navigation + some darkness. Fastest 30 is 5:42 again all trail + navigation.
As has been said you can lose a lot of time in the last 20 if you go out too fast.
I can't run at 12mm it is just too slow. In fact I struggle to have much of an effective run slower than 10-10:30mm. Steady walk for me is 15mm and powering along on the flat I walk at 13:40mm.Anything inbetween is netiher an effective run or a comfortable walk - the run just seems to waste energy for little extra forward motion. So can you actually run this slowly?
So my first question is - how fast is your walk. Then consider walk/run ratios to achieve goal pace. I can handle 12mm on the second half of an Ironman marathon which is basically walk 7:30 minutes, run 2:30 minute (or 4/1 ish)
The walk breaks also give you a great opportunity to drink/eat - definitely think about eating/drinking on the move. It saves a lot of time and stops you stiffening up. Save the stopping time for sock changes and/or trainer changes if going round a loop.
I find it quite easy to pick up pace in the last 20, I am bored with walking so run for a break. I wouldn't say I sprint for home but probably manage more jogging in the last 20 than the first 30.
Note: Walking does not mean moving at shopping speed!