If this his sort of event is relatively new to you, then some of the following advice might be useful and a cyclists nod of acknowledgement to the folk at the London School of Cycling, whose Dunwich Dynamo text we have used for the basis of ours.
Although it’ll be mid-summer, you’ll need decent lights. If you click HERE you can read a very good review from Cycling Weekly about different lights and options. It's a good idea to have back up lights with you as from experince we needed the by the end as the batteries had run out on our main front light.
Don't overdo it for a couple of days beforehand.
Give your bike a good clean/lube and check the tyres for bits of glass etc.
Check the weather forecast and consider the coldest predicted temperature. Take an extra layer of clothes if you think you will need them.
It will be dark so wear bright or high vis kit.
Chamois cream or some other skin balm may be useful on your bits - use it before you get sore.
Drink a lot of water, don't dehydrate.
Don't race off, settle into a pace you like. (you’ll hopefully meet several new people, ride together, and find a perfect pace that suits a few people....and take turns at the front to keep it going).
Carry spare batteries - if you don't have a dynamo - a couple of inner tubes and a patch kit.
Take a snack or two for on the road. There’ll also be food for you at the chippy in Moffat if you want it and bacon sarnies at the end.
A small towel might be nice if you plan to have a swim!
Don't eat too much at the stop, you have the climb starting straight afterwards!
Carry some food, aim for self-reliance and be prepared to offer support to any distressed pilgrims you meet.
As this is a totally unsupported event it is probably worth considering doing this in a group or a the very minimum pairing up with a pal. That way, even though there will be other cyclists on the route, you can watch out for each other.
Don't let the thought of the distance get you down...we can all do it and will do it.
It's a beautiful ride, enjoy it.