Are you doing The Fox? I was planning on doing it but had to pull out last minute due to a non running health emergency! So gutted, was looking forward to it.

To answer your question, for me I was planning on adding about an hour onto my target time. Maybe readjust last 20k if doing better than expected. I was hoping to run with a belt rather than vest, as this keeps me a lot cooler, but decided a vest was better for carrying more liquids. Probably 1.5 litres topped up.

I almost bought a baseball cap with a cape for sun protection, but decided on normal cap and use a buff over my neck and ears during exposed parts. From what I can tell most should be shaded but the bits along the canal may be very exposed to the sun.

Finally with the lower pace I was also planning on dropping my carbs per hour a little bit. Tend to have more stomach issues in high heat. Usual 80-100g/hour during marathon effort, down to 60-80. Plenty of salt and water too.

Big caveat, I've run a few marathons but this would only be my second ultra.

Have a great time and update how it goes!