Newlywed Ryan Reynolds isn't just sitting around enjoying married life with bride Scarlett Johansson -- he's waking up at the crack of dawn to fulfill a momentous challenge -- running the New York marathon!

In a new essay for the Huffington Post, Ryan writes that he's not naturally inclined to run a marathon: "I am not a runner. I'm a running joke." In fact, a few years back he watched the finish of the New York marathon from a comfy vantage point near restaurant Tavern on the Green, and remembers: "People were crying. People were limping, hobbling, screaming, crawling. But most importantly, people were experiencing a sublime rapture that I couldn't even hope to understand."

But that didn't convince him, "At that moment, with all the energy, inspiration and passion swirling through the crisp, autumnal air, I breathed in deeply and decided something: I'd never f***ing do that ever, ever."

So what changed his mind? Ryan gets serious and writes that his dad, Jim, suffers from Parkinson's disease. In a conversation with Parkinson's sufferer Michael J. Fox, Ryan recounts, "In a moment of extremely poor judgment, I offered to do something big for him -- I offered to compete in the New York City Marathon. Which was, well … dumb."

Ryan, who promises to "sob uncontrollably" during the race, says that no contribution is too small (or too large) so if you'd like to help the fight against Parkinson's, you can contribute on his donation web page at Michael J. Fox's Team Fox for Parkinson's Research site.