24 Hour ride

Later on this year I hope to ride 320 km's in 24 hours on a mountain bike with slick tyres.
I have a few questions:
Firstly, what time of day should i start? I am presuming the ride will take around 24 hours as the course is extremely undulating and a high average speed is difficult on a heavy bike with a relatively unaerodynamic riding position.
Secondly, how often should I stop to eat/rest? Usually I find a long rest, i.e. greater than 10 mins is not beneficial.
Thirdly, has anyone else experienced knee pain when riding for 12 or more hours with only minimal breaks? How do you rectify this problem? I have previously attempted this distance but had to quit due to excruciating knee pain, even when I wasn't pedalling. I always spin a very easy gear when on rides of this duration (approx 90-100 rpm), but still found my knees ache. I was riding in 10 degree Celcius temperatures for the first two hours of this ride, would this affect my knees considerably?
Any other suggestions or comments would be appreciated.
Thanks