As someone who enjoys the temptation of a couch-tv-pizza-beer evening, I have to say that in recent years I have discovered the benefit of keeping a mind-body balance. I started running three years ago and started regular weight training two years ago.
Don't get me wrong, I am still not all that athletic, nor do I look like an extra from "Spartacus" but I have lost some weight and gained some fitness, but mainly what I've gained is increased confidence, peace of mind, boosted mood, more focus and a lot more creativity than I possessed in the past. I would like to think I had the creativity, but I needed to get in balance to really access it.
Of course I still read a lot, check out movies and video games and comics - basically all the stuff that fuels my fire to write and create in the worlds of GGE, but I make time to see to my fitness as well.
I've seen huge benefits. I haven't suffered too much pain, but there is always some scheduling nightmare to getting to the gym or going for a run. Make it a priority though and it will fit. Just like writing.