Running

That's great! Good luck with your upcoming 80km.

From my own experience (~3 years of running, 1 100k, 7-10 runs between 50 and 70kms, 4 marathons, and a couple of shorter distances recently), consistent speed work is important, and goes a long way for running longer distances. I still run at 60% heart rate, but the speed is higher and the races become easier.

I would do a marathon if you want to. I found them fun, but they are not for everyone. A very different experience from an ultra.

Re hills, I find downhills much harder and more important to train for than uphills. The time you lose from walking uphill is minimal compared to running.

First of all, all the best for your training and 50miler to come! I'm in a similar boat with you, exploring trails vs road marathons the past few years. The main experience I can impart is prioritising the immediate race to come, and parking the marathon and/or TORX at the back of your mind. Spending minimal time and effort thinking about it. This ruined me in the past where I tried to dovetail trainings and races over the year, and flopped in both.

Secondly regarding the slope, weight trainings really helped me. Such as squat, lunges, step ups, and gradually with weights. This built strength and endurance in my legs, knees and ankles. I do this about once a week, with the remaining 50-70% training on easy runs, and another 1xhills. Good luck!