Preparing for a 40mile road ride... What do I need?

So as part of my loosing weight getting fit my wife has signed me up for the London to reading ride with the British heart foundation.

Great for helping my fitness and hopefully loose a little weight too!

I'm planning on fitting an outer chainring for flat/downhill sections and re-fitting the dmr moto RTs and running them at 50psi+ to help with rolling resistance.

What else will I definitely need for a constant 3-4hours in the saddle?

I don't normally worry for a normal ride as they generally end up with a fair amount of time out of the saddle and also a fair few stops to chat/eat etc.

Any advice/tips welcome.

a sick note from you mum!!!!!!!!!!!!!!

A comfy saddle, that you're used to.
Padded shorts.
Bar ends to change hand positions.
Padded gloves and grips you're used to.
Mudguards if the weather's foul, or maybe a crudguard up front 3-4hours of crap in face will ruin your day.
2 bottles or plenty of fluid in a bak.
A couple of snacks.

I'd think 4hrs should be easily do-able for that distance, comfort will be the main thing.

Padded cycling shorts are on the list.

I ride with a charge spoon which I find comfy.

Camelbak will have my fluids in. Then just need to sort some food for the journey.

I've no problem doing 20mile rides just need to try a few longer distance to see how I get on.

Just ride at a comfortable pace and don't push yourself too hard, too often or for too long. Do that and you'll be suprised how far you can go. It's very easy to get carried away and wear yourself out early on.

If it's going to be on a warm sunny day, perhaps take some form of Electrolyte-replenishing drink or tabs with you.
Nuun are my fave, something of a life-saver in summer here.

A road bike

At the very least, some proper narrow slicks pumped up hard.

20 quid for a few pints at the half way point.

It certainly won't be warm, it's in March. Probably a balmy 10 degrees, if he's lucky.
Ride with a bunch, a bit of a laugh and a natter will help alleviate the boredom of forty miles of tarmac riding, plus you can take turns at the front which will push your speed up a bit, especially if you run into any headwinds.
Don't go mad for the first few miles, use them as a warm-up, and then enjoy the satisfaction of passing people who've set off at too fast a pace for them to sustain.

Something warm to put on afterwards(lightweight down jackets are superb for this and it packs really small and weighs little.
A treat is a nice thing to have at the end,something like a marsbar or even a nice flask of piping hot soup or even coffee