Hi, I've run this 3 times now and can vouch for it being a truly memorable experience. The weather is generally atrocious, but the views spectacular. I always take my family and we make a weekend of it, stopping in one of the nearby hotels.
Basically, you start with a 3 1/2 mile climb to Pen-y-Gwryd and it's best not to push too hard or you will blow out before you've even got started. At the top of this point, the views down the valley are fantastic and you can enjoy 8 miles virtually all downhill - sheep in the road, waterfalls cascading down the mountain forming streams across the roads, etc., it's brilliant. At this point you turn at a lovely Welsh town/village called Beddgelert, where you get some encouraging support. Then it's fairly flat until around 21-22 miles, although the views of the mountains and the lakes will take your breath away. The next 2 miles are a killer, there's no getting away from it, but then you're close to the end and so who cares. Most walk this part. Again, gorgeous views as you climb to the summit of this point, but then you look back down to the finish line in LLanberis and it's a rapid descent down into the town !
I've run London 7 times, but Snowdon is soooooooo much more rewarding. Even talking about it brings on those pangs and I'm thinking of doing it again this year now (I try to do it every 2 or 3 years).
My fastest marathon time for Snowdon has been 3-45 and this compares to my London time in the same year of about 3-30, so it's not that much slower. You can certainly make up a lot of time from miles 4 to 12, as long as you haven't overdone it in the first 4.
For anybody thinking of doing it, you should if even once. Don't be frightened of the title "toughest marathon in Europe", they're all tough and you pace yourself accordingly. It's a very proud one to have on your cv !!!
Kevin