Well at 4.3W/Kg I think your 10 hour mark is very very conservative.
The main factors that affect the time other than your own fitness are the weather, the group(s) you ride with, your attention to detail on your feeding/aero/support etc etc.
Here is a link to the TrainingPeaks file of my 2017 ride which was off an FTP of around 4.2W/Kg (63Kg/265W)
Fred Whitton 2017
There were a combination of factors which led to that time but I made it my season goal and managed to nail it. One thing I would highlight on that ride is that I only stopped at the summit of Whinlatter very briefly as the guys I was riding with didn’t have dedicated support so needed to stop to grab their food/bottles. I think its on the videos I linked up thread. Apart from that it was constant movement.
Feeding wise I have been spoiled for support so I dont use the feeds provided.
i have the first at the summit of Kirkstone, one on the A66 approaching Keswick, one at the summit of Whinlatter, one near Santon Bridge and a final feed at Cockley Beck.
This enables me to run very light with one bottle and little in my pockets. Without support you need to be running two large bottles and plenty off food/gels.
My feeding strategy is;
1 x Rice Cake following descent of Kirkstone
1 x Bar on A66 on the fast stretch following Scales Farm
1 x Rice Cake Borrowdale
1 x Gel on the descent of Honister
1 x Gel on the descent of Newlands
1 x Bar after the descent Whinlatter
1 x Rice Cake on the run to Ennerdale Bridge
1 x Rice Cake on the descent of Cold Fell
1 x Bar after Gosforth
1 x Caffeine Gel at King George V pub (about 20 min from Hardknott)
1 x Gel at Cockley Beck
On top of that I get through 5 to 5.5 small bottles of energy drink (Torq in my case) different flavour in every bottle. Bland non sweet flavours to start.
With regards to groups, if you dont have a group to ride with prior to the event it may be worth hanging around the start and looking for groups in club kit setting off. Kendal CC, Honister 92, Harrogate Nova usually have decent bunches of riders setting off. I’m in Kendal CC and they usually go off around 7am to 7:15am ish in the past. Not sure what there plan is this year and I am not riding. If I hear I will send you a message if you like?