In reply to johnboybuchan:
For me the biggest difference between 100km (60 miles) and longer distances is that I need to get two things right - feeding and pacing. Feeding - on shorter distances I can get away with being slapdash - on longer rides you really pay the price if you don't eat enough. Obviously the faster you are trying to go, the more critical it becoems. As Bob says, start eating early, and keep eating - I try and eat at least one substantial piece of food every hour, and keep snacking on a pocket full something that I can just pull out and stuff in my mouth - I like dried mango. Are you happy getting food out of your pockets, unwrapping it, and eating on the move, while still going in a straight line as part of a group? If not, getting comfortable with doing this is something you could profitably do in 3 weeks. If you can't, you'll have to keep stopping to eat and you won't want to do this if you are making good time as part of a group. Same goes for drinking only more so.
What are you planning on eating? I would get this sorted in training so you aren't experimenting with something new on the day without some idea of how your body will react to it. Personally I find that I get sick of eating very processed sports nutrition products (bars, gels, etc) after a few hours on the bike and really struggle to force it down. I find food I have prepared is much more appetising. I like rice cakes, this is a good recipe: current favourite is dried apple, cinnamon, and some posh mix of raisins/cherries/cranberries to keep it interesting.
Also really recommend this book for a bit more variety, lots of the recipies are veggie or can be made so:
http://www.amazon.co.uk/s/?ie=UTF8&keywords=feed+zone+portables
Drink wise - know how far it is between water stops. I always take two bidons might not always fill them both if I know it is not far to the next stop - saves having to lug extra weight especially if there is any steep climbing to be done. Might as well point out here that if you are concerned about time, the easiest gains are the time you might spend standing around at a feed zone - it is really easy to lose 20 minutes per stop without really noticing. If I am trying to get a fast time, I try to be selective about where I stop, and make stops really fast - go in, have a wee, grab food, fill water, get back on the bike - then eat/drink once you are moving.
Pacing - I find 100km rides I get away with going out fairly quick (eg. hr zone 3) but struggle to keep this up for 160km/100 miles. For longer distances I try to ride at an easy pace (eg. hr zone 2, or easy enough you could maintain a conversation) until halfway, then up the pace if I am still feeling ok. You probably have some idea how fast you can sustain 60km, so once you reach 60km to go you can afford to press on. Concentrate on trying to be really efficient with your pacing and don't let yourself go over threshold. On my first 100 miler, I got onto the back of a really fast moving group and made really good time, but should have had the sense to let them go earlier than I did - eventually I was going into the red just trying to stay on the back, I got dropped on a hill and then had to crawl a really, really painful solo 20km to the next feed stop bonking the whole way. This is probably the worst 20km I have ever done on a bike.
I find an HR monitor useful to pace myself, especially to make sure I am not working too hard too early on - on a sportive it is easy to get carried away, you think you are not trying that hard but in reality you are. Then again it only really becomes a useful tool once you have used it in training and you know what e.g. 150 bpm actually means in terms of how long you can maintain it for, otherwise it's a bit meaningless, so if you don't have one already it might be a bit of a distraction.