Multi-race Training Schedule

Hi, I'm new to the forum and to racing. I've been a recreational runner for years and have dabbled in longer distances from time to time, but this past year I completed a trail half marathon and a 25k. I'm pretty ignorant about longer-term training in terms of injury prevention. I'm a naturally big guy. I'd like to tackle a trail marathon this year, re-evaluate and then consider a 50k option. My question is, if I were shooting for a 50k, would I just plan a trail marathon in the same schedule on my way to a 50k, or would it be better to train and race the marathon, break for a period and then do a whole different training for 50k after that. Again, I'm new to this so any information or guidance would be great. Thanks!