As you are doing the ride today, I'm probably too late with this but here goes anyway.
Although you describe the road as rural, I've had a look along part of it on streetview and it's not some idyllic country lane. If you have the confidence to use that road, then this junction shouldn't faze you. The ideal line, as has been recommended above is to do the same as motor traffic: ie, you need to be in the outer, straight-ahead lane to go straight ahead. It's not going to get any easier moving out there nearer the junction, when you will have to deal with left turning traffic while trying to join your intended lane, so the ideal place is where the road widens to form two lanes; just don't give anybody the slightest reason to believe you may be turning left. However, a lot depends on the speed of the traffic approaching you from behind. Even a professional racing cyclist has only so much ability to match the speed of motor traffic. There's no recognised "I'm going straight ahead" signal so, unless the road is clear behind, a clear "I intend to turn right/ change lane" signal is the only recognised signal available until you are clearly in the straight ahead lane.
A lot depends on the state of the traffic lights as you approach. If they are at green in your favour, drivers behind you may anticipate that the inevitable gap in the traffic in front of you will trigger a change of the lights to STOP and they may try to pass you to avoid being held up. You need the confidence (courage?) to hold the centre of that lane. If the lights are already at red and a queue has formed, it may be advantageous to get near to the head of the queue, if you can filter safely, bearing in mind that the lights may change again before you get up there. There are different opinions about where to position yourself if you do get to the front with the lights at red. There's no advanced stop line (ASL) so it's illegal to cross the white line, even to stop just over it. Not everybody complies with that. An alternative is just behind the first vehicle in the queue. (This is one of those locations where cyclists should be allowed to treat the traffic lights as GIVE WAY signs, to get across the junction during gaps in the traffic but it's illegal so I'll not recommend it.)
None of that is much use if you lack the confidence to take the correct lane. In those circumstances, you can only improvise, depending on conditions. It
might be possible to use the left turn lane and somehow get onto the verge just before the junction and wait for the lights to stop the traffic. Then you could ride out to your correct lane. Or, has been suggested, turn left then rejoin from the side road. Neither is ideal, but if you cannot get out into the correct lane you need some sort of alternative.
I'm conscious of using the word "depends" but that's how it is at locations like this.
There's lot's of good advice in "Cyclecraft."
http://www.cyclecraft.co.uk/(My only connection is as a satisfied reader.)