I'm thinking to maintain 15mph for 40mile stints for 24hrs I should be hitting 80-90 RPM to keep the load/stress less?
Try not to have a fixed speed in mind when you're doing this.
At the start, when you're fresh, if there is a slight climb or small headwind it can slow you down without you realising it, so you might try to push too hard to maintain your pace.
Also, you'll naturally tire out and get slower as you go so if you're dropping below your target pace after several hours and you try to push a bit to maintain the pace, you'll probably blow up.
Manage your cadence and heart rate and if you're going faster than expected, that's great, but if you're going slower, dont panic about it.
Breaking into 40 mile chunks is good but be careful with eating/drinking. If you're 35 miles into your 40 mile chunk and you start to feel hungry, it would be easy to ignore and just wait 5 miles to stop and eat. It's not a good idea to do that though. You dont have to stop and eat loads, just a banana or something to get you to the next stop.
Let me know if you have any other questions. I'm no expert, but i'v done a couple of 24 hour races so i know a little bit about what to expect.
When is your ride?