Pondering this exact question caused Canadian science fiction writer Robert J. Sawyer to postpone working on his current trilogy, WWW, so he could write Rollback, where an elderly couple is given a super-expensive youth restoration procedure so the wife can continue her work on an important scientific problem. The husband only gets the procedure because his wife insists she won’t do it alone – but sadly, the procedure doesn’t work for her, but it works fine for him. So he ends up as a young adult married to an octogenarian.
While I agree that the “out of your league” thing is stupid, you aren’t really hitting on the reasoning. It’s that the amount of effort and commitment involved is minimal, and thus the amount of pain is also minimal.
There are plenty of things where the bad side of stopping outweighs the good of starting. Heck, it’s the best argument on drugs, I think.
That said, in this situation, I don’t think that happens. Nothing about the situation seems inevitable. If you’re immortal, what’s wrong with staying with someone for the rest of their life?
Also, I wonder if it’d be possible to salvage the virus from her, and make myself immortal. Then again, I’d rather be Enoched instead.