I'm doing the Fred route later this year (not on the day of the event). I know what to do to get up the hills - train a lot and lose weight. But I'm a cowardly descender, so getting down should be even more of a problem. Might have to carry a spare set of brake blocks!
You do right taking it easy on the descents. The roads are rippled, gravelly and as soon as you let go of the brakes you are instantly doing 20+mph. Every year in the Fred there are numerous accidents, mostly on the descents.
If it's wet the the issues are compounded. Last year I went over the top of Wrynose going towards Hardknott in the rain. As the speed picked up I thought "Hello, time for some brakes". As The rims and blocks were wet it took a few seconds for them to sweep the rim and start to slow me down, by which time I was going a lot faster than I wanted to be going.
There was a serious pinch mark in my saddle I can tell you!
I found when I did the Fred the Cold fell bit is the toughest, it's usually into a head wind, there's no real 'named' climbs but all the little rises take digs out of the reserves and seems to go on for ever until you get to Eskdale and so when you hit the last big push at Hardknott you do have to dig deep. It's a great event though.