Anybody recommend a good training plan or a place to find one that would be suitable?
Nothing complicated necessary. It is too dark and cold to go long in the winter, so go hard. 3 FTP type interval sessions per week is enough. Workouts on the trainer can be very productive. 2 X 20 min with 10 min easy warm up and 5 min easy between the intervals. You can get a bit more complex by doing some 5, 7 or 10 min intervals as well. Probably does not matter much. Just ride hard.
Some would recommend slowing down to “build base.” However, if you slow down and reduce volume you get less fit, not more fit. Fitness is about progressive overload.
Depends on location and your full year plan. When it is actually cold (near or below freezing), I find it difficult to do anything other than endurance rides outside. Whether daylight is an issue depends on A) your job B) where you live C)what sort of lights you’ve got. I ride early and light up like a christmas tree. I like riding when it is dark outside and no one is on the road yet.
Planning wise you have to decide how you want to structure your rides: normal periodization or reverse. I’ve heard it suggested that reverse might be better for Ironman type training, as your specific stuff is going long it should be your build block. I wouldn’t know, I haven’t done one.