How to prep for a non-official marathon?

Tomorrow I’m going to run a marathon length type of run/jog. Im in decent shape but I want to see if I can handle it since today I almost went 8 miles in 2 hours which for me is pretty good since I’ve never done that before and I was walking the whole time. If I’m getting burnt out I’ll go a half marathon. But what do you recommend for preparation. Im pretty confident I’ll be able to do it since I’ve got all day so not much pressure to finish before a certain time
How to prep for a non-official marathon?
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