The SBU is a pretty straight forward route, with only 2 major climbs, Honister and Dent towards the end, with the remainder being mainly flat. By far the hardest bit of the course is the descent from Honister to Buttermere (I think).
When I ran it a few years back, the course was not marked - something to be aware of, if your nav. isn't that great - and you can spend a fair amount of time puzzling the roadbook instructions.
There are only a few sections which are tough underfoot as TRex describes, the remainder is a mix of fire road, footpath and some road.
Nice finish location at the School in St Bees and a free beer to top the day off, what more could you want?
As with all training, specificity is the key. You have two challenges -
Running 6 consecutive days and recovery.
Terrain and climate.
Your training needs to reflect these as best you can. I'd suggest building up to running 3-4 days on the trot regularly, with an occasional (once a month or less) week of 6 days running. The structure of these days is really up to you, but try and keep some quality in there.
Not sure what the terrain on the Coastal Challenge is, but try to simulate this as best you can. Same with climate - many layers on a warm summers day maybe?
There's no shame in a DNF for any reason imo. You tried and weren't equal to the task on the day. The hard part is figuring out why, and then addressing those issues so that it doesn't happen the next time.