I think you're being very generous with the times.
That's not just me saying that cos I'm a 'geek', the L2B average is quicker and you can't pedal for some.
Plus 6am kick off, it will still be dark.
It is essentially down hill, (until Ditchling).
The average speed should comfortably be double figures.
For reference the Gillingham ride will have been much harder.
10 mph would make it just under five hours SB. 12mph should be achievable by most but you are only as fast as your slowest person in situations like this, and you know full well someone will turn up on a full suspension Apollo from Halfords