My bikes have low gears of 40x32 on 27x1-1/4, and 28x30 on 700x28, which give me about 25 g.i. and 34 g.i., respectively. 25 is fine for me; 34 is OK, but there are some hills I do avoid.

I wouldn't even think of trying to go up a real hill on a 42x24. If I tried, it would end my riding day. I would walk. I have old knees.

I'd go for a 32T big cog with that 42T front, if it were me (which you're not,. of course). You'd almost certainly have to change the rear derailleur for a 32T, though.