Book Description

I think the term "multi-century lives" when coupled with time travel in the same sentence may confuse some people. The term might mean they're already time travelers ( a couple who spend their lives in multiple centuries) if taken out of context. "Super-human life expectancy" is more refined and explanatory but far, far more convoluted to say. Especially as you mention their long lives in the second paragraph (repetition) which might compound the idea in the head of anyone who mistakes it the first time. I think rewording that their lives are superhuman at the start of that second paragraph and replacing the first sentence with something else might help it.

I can see you're trying really hard to avoid spoilers, JD, but I honestly think you need to mention the middle ages or more strongly hint at the period or location of Europe to give people a flavour of what they can expect. At present, it feels like an idea of a story on a page, when as a reader I want that flavour of what I can expect that makes it distinct.

This says to me
"Time travel story,
a privileged couple with no resources,
they must solve the time-travel mystery to get back.
Can they learn to work together, avoid changing timeline, get back?" Which sounds very generic.

It needs to say more of
"Time travel story,
a privileged couple stuck in exotic medieval times with no resources,
struggling to adapt to bizarreness!
(not they must but) CAN they even solve the mystery of how they time traveled? It seems impossible!
Can they learn to work together, avoid changing timeline, get back?"

I think in short the word im looking for is juj. It needs jujjing up, ramping up the excitement and impossibility of the odds against them, as well as that flavour of the era.

The more you avoid all spoilers the less interesting the book sounds. Here's the blurb I googled for Back to the Future from a VHS or DVD.

Adding the period is kind of important since it very much sets the theme of the book. Knowing BttF is in the 1950s doesn't spoil the movie when you watch it for the first time.

That's half of the first book written out!! hehe
(it has no real bearing on yours, but I thought it'd post it to prove I think you can get away with revealing just a little more and not call it spoilerish.)