[Middle-aged, slow, stubborn, dogged and independent]
It will be fine. Really. I ran a marathon distance not having run more than a half-marathon as my longest distance beforehand. You know this already but: take it steady, keep to your cadence and expect everything to ache afterwards.Oh, and the very best of luck.
...and not go off like a rocket after the bike because everything always feels slow at that point. Manage the effort. The theory is all there, but the unknown of it is hard for a 15ish hour "race".