Easy chap!
First off, 17km ain’t a long run 
Wrap your head around that one first. 
(Joking. Of course it is. But add another 25km to that 
)
Is that your longest ever run?
Have you a ran marathon before?
They’re miserable things.
Just try to enjoy it.
Run at a pace you feel comfy with and ignore the numbers.
Repeat again next year. And again. Until you succeed.
The end.