How hard is it for someone who can fly model helicopters reasonably well to learn to fly fixed wing?

with an instructor and the right plane, it's not hard at all, but going by yourself is a totaly different thing. at the top of the forum is a couple of threads for beginners that will help a lot

http://www.rcuniverse.com/forum/m_4537845/tm.htm

http://www.rcuniverse.com/forum/m_8503667/tm.htm

Dito on all rgm762 said. You already have the basic skills and muscle memory needed, the most important of which is being able to reverse some control inputs when you are flying back toward yourself. If you can hover "nose-in", you won't have any problem getting a plane to fly back towards you. In addition to an instructor, I would also recommend a simulator. I went the other way, started with fixed wing and have now learned to fly helicopters. The simulator was a valuable tool, especially with the help of an instructor who instructed me while flying on the sim.

We've had two helicopter pilots come out to our club this spring for flight training on fixed wing aircraft during our regular training nights. Both pilots were solo'd after just a few sessions.