Welcome to the club.
There are a number of different ways to fix that. I would probably dig out all the soft and rotten wood I could first. Then mix some bondo with a universal tinting color to match the lighter color of the wood and fill it with that. Then mix some paint to match the darker color of the wood and draw the grain of the wood on with a small artist brush. There are also epoxy products made for that purpose however like the link but I don't have any experience with them. With them you might have to fill the void first and then paint it the lighter color of the wood and then grain it.
http://www.homedepot.com/p/PC-Products-Rotted-Wood-Repair-Kit-084113/100664090
There are a number of different ways to fix that. I would probably dig out all the soft and rotten wood I could first. Then mix some bondo with a universal tinting color to match the lighter color of the wood and fill it with that. Then mix some paint to match the darker color of the wood and draw the grain of the wood on with a small artist brush. There are also epoxy products made for that purpose however like the link but I don't have any experience with them. With them you might have to fill the void first and then paint it the lighter color of the wood and then grain it.
http://www.homedepot.com/p/PC-Products-Rotted-Wood-Repair-Kit-084113/100664090