I'm sure you can handle it just fine, I just finished my MR fron BUF-BOS-LAX-NRT-SIN-NRT-SEA-ORD-BTV-BUF, my flight was actually much longer than 15 hours. I think as long as you can find something to think , or to sleep, to watch the PTV, things will turn out fine, get some books you want to read.

^