Absolutely, gear down if you feel uncomfortable in that gear. You will actually be able to sustain that speed longer. I'd suggest, trying the 17T first, and seeing if you can maintain 90 rpm w/o getting tired. Eventually, you can return to the 16T, once you gain strength and endurance. FWIW, I ride between 38 x 16 and 42 x 16, which is roughly the same as 42 x 18 and 42 x 16.