They just need to ride their bikes as much as possible, 100 miles a week should work well on MTBs - how old are they? Jelly babies, malt loaf, cake while doing distances...avoid booze, get used to how much liquid they need, get plenty of rest. I don't think it's a hard ride due to all the other bike traffic. A friend of mine did it on an MTB no bother and she really isn't much of a sportswoman although I expect she'll destroy me when we do the London-Surrey 100, she is helped by being 45kg lighter and 10 years younger though!