I was told by an old hand to regard each loop of the ride as a new ride. He said trying to think of 50 was too stressful when you first start!
The other thing is to try and do each loop in the same time. So if you do the first 25 in 2.30, try and do the second in the same time. It’s really important to be able to gauge the speed you’re travelling. If you have a gps you can check on the speed.
We do our 25’s in 2.05-10, but if we’re doing a 50 I slow right down to 2.30 - 35 for the 25, depending on the terrain.
Most of the time I’m more tired than my pony, at the finish. lol.