The key here is, as many people have said, FUN. I read a quote somewhere, may even have been on this site - "If you're not having fun, get out of the water".
BUT - I don't mean you should give up, not at all! My point is you should stay where you are comfortable and having fun. If you surf whitewater for the rest of your life and enjoy it then that's your decision. You don't EVER have to go to the line-up if you don't want to.
My advice is to stay in the whitewater until it doesn't bother you at all. You should be able to stand, turn, jump off and generally mess about without being afraid of the waves before you try for the line-up.
When you do decide to go for it. Pick a SMALL day - about 1 - 2 foot to try, this way the jump from whitewater is not too big.
As regards duckdiving, get the basics down in the whitewater first. Duckdiving is a hard thing to get right and you WILL get hammered when you are trying to learn. I duckdive my 7 ft 5 mini-mal quite easily. I've seen someone duck dive a 9 ft 6 longboard, so it can be done.
If you're getting frustrated and want to get to the lineup, but can't duckdive - try this. Firstly, take time out to watch the sets. You will notice that the waves come in groups of about 3 - 7 waves. Then there will be a lull. Try to be as deep as you can stand comfortably at the end of the set. Jump/ duck the last wave, then get on your board and paddle like crazy to get out before the next set! It sounds stupid but this is what the pros do!
IF you get caught inside, this is my solution. Get off the board and tread water. point the board towards the shore with the fins UP. grab the leash near where it attaches to the board. duck under the wave and pull down on the leash, the wave will go over you, but you won't have much time to get back on and paddle again.
If you need to, just repeat the above til the end of the set, then get back on and paddle like crazy again.
Paddling is the key to successful surfing!!