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Do it. You'll be fine. I've found that jumping into up to 25k is no problem off of regular old training (which for me is 2 trail shuffles a week, plus a bunch of bike miles). Did one 50k on gravel roads, that was much more painful and I'd recommend actually training for that. Plus there's usually whiskey at the trail run aid stations, which helps immensely. The variation in stress on your body from dodging rocks/roots/etc will tear you up much less than pounding a pavement marathon even though it takes longer.
N=1 and from a running background, so take this with a grain of salt. When I'm in sub-20 off the bike shape (it's been a while), my open half is usually in the 1:20s. You don't really need more volume, just do a couple of quality interval/tempo sessions a week. Price-is-right prediction is 1:25:02.