I've done long rides in the past, the longest of which was 259 km less than 2 years ago, but I've fallen very much out of shape.
I am currently overweight, but I know how to solve this part and by the time I want to do the ride in question, I should weigh around 85 kg. I'm 184 cm tall as a frame of reference.
I would like to do a 280 km ride in May next year, with elevation gain probably exceeding 3000 m - hard to find flat routes where I live. Currently, most rides I do are around 25 km long - I think I could manage a 60 km ride now, but it wouldn't be fun in my current shape. What are the steps I can take, apart from losing weight and just cycling, to ensure I'm ready to do this come next May?