I’ve not ridden Selkirk but if it’s anything like the 1st one at Builth Wells expect lots of ups!
Training wise don’t worry to much on the distance you cover, just get time in the saddle, if your new to marathons and expect to do the 100km course you need to be prepared for 6+hrs in the saddle. Food and fluids at every opportunity, I rode the ELBNO and then the 65km marathon last time and because it was hotter than expected didn’t get enough fluids in early enough, so after 3hrs I started cramping not good!
Enjoy the atmosphere, don’t get sucked in to riding faster than you comfortable with too early, it’s not a race, and be prepared for a great day out!
I’ll be there hopefully last big ride before I hit the TransWales!