I've done this three times in the past with my wife and various friends who do and don't cycle much.
The tricky climb onto the north downs ridge has been levelled with gravel and The long drag is a drag on the old railway line so it not that technical - the distance is what catches some people out.
I would take a HT, have loads of pressure in the tyres - which you can let down for the final drop in the Shoreham, Take food, think about the trip back on the train to London if that's your thing (faff /food/ water / lights if it takes you longer than you think, as there is some urban riding to negotiate at the end, or on the way home from the station).
And finally - don't race people, as there are going to be people doing this for a real life challenge, and the distance gets to everyone who kicks on at pace at the start. As has been mentioned the climb on to the south downs then up to the YHA is mean if you are low on juice.
Enjoy - it's a great day out. Go Raise money.