I would concur with the advise already given.
From my experience apart from the details about the stages - you need to assess the strengths and weaknesses of your team.
Some people will be good "rouleurs", others will be good climbers.
If you are acting as a domestique, you need to discuss with the rest of your team - the role they expect you to provide for them.
What are your strengths as a cyclist ?
Will your team be expecting you to perform as a climber, when you're possibly more suited to cycling on the flat or vice versa ? (not knowing your background, I cannot give you specific advise).
What I can tell you is that I raced (years ago) in major amateur stage races and a five day event is significantly different to doing one day races (like Tom Davis said).
You need to have a very good level of fitness and be able to cycle
at a minimum of 23mph for up to 4-5 hours.
That is the minimum standard at national level here in Ireland.
Seeing that you are just below elite level (in the UK ???), I suspect that you may have the fitness level required but I cannot be certain.
We currently have the Ras going on here : this race was one of the toughest amateur races in the world.
It has now gone up another notch, now that it is open to professionals (Malcolm Elliot - the former British professional and grand tour rider is racing in this years event).
The average speed is up at around 26-27mph.
Without having specific information about your condition and your team, this is about as much advice as I can give.
if you can pad out some more detail, I will be happy to pass on any advice that I can.