I've run this loads of times. There is a short sharp climb after 2 ish miles, then a mile or so later at Stair, it goes down sharply. At 4-5 miles at The Swinside it goes up, but it'snot the end of the world. At the foot of Catbells, it goes up and then up again and this is the end of the world. The last few miles along the Borrowdale Road are a bit gutty, but all in all this is one of the best races in the world. Honestly, this is a must do race.
The heart and lungs adapt quite quickly to running distances. However, the joints, muscles and soft tissue take much longer and thus you will be prone to injury. Just be careful in your build and you should be OK. A good estimate for your marathon time (subject to lots of things) is 5x10k time.
I have copies of the magazine when it was called Running. Back in the mid 80s, it wasn't a bad magazine at all and a load of interesting articles. In fact, I might have copies of when it was called Jogging. All downhill since the Rodale took over and repositioned it in the market for beginners and dieters.