Firstly, this is me: http://www.bikemagic.com/news/article/mps/uan/6845
If you want my advice (lots of hours logged on long SDW rides), if you've done several 3-4hr rides and are comfy, a careful 100 is easily possible. If you've done lots of 15-20miles rides that have been out of the house for 4hrs but chatting with mates at junctions and gates (this second option was me), get some longer rides in with fewer breaks first
Useful things are:
1) Two bottle cages, not a camelback, get the weight off your back (you'
ll appreciate that after 10hrs!)
2) Get some sudocrem, or similar, chamois cream for your bum
3) Take some electrolites (my error) to help muscle and stomach cramps stay away. Bananas are good.
4) Start eating/drinking in the first hour. your body can only process 300kcal of carbs an hour approximately, you'll be burning about 800 an hours, get it working from the off to avoid it having to break down crazy amounts of energy from you.
5) You'll need to drink up to a bottle an hour depending on the heat
6) Don't over eat, this can be as fatal as eating too little
7) Don't change ANYTHING on your bike the night before, don't use anything on the day you've not used before.
10mph average quoted above is tough, specially on the BHF ride with lots of other riders. It's NOT a race, it's an endurance challenge. Look at the finish times from last year, you'll see what a reasonable time is.
9) Don't fit skinny race tyres, don't pump your tyres up too hard. I use 2.2" tyres, tubeless, at normal/low-ish pressures for comfort and grip. Your back will not thank you for going with "faster" tryes
10) Smile and enjoy the fantastic views and scenery! You'll have a great day out.