Doing all of 75mpw on 400m road

Hey mate

No it won't have a major affect on your body, as long as you're not always going in the same direction. As you said, at least you can still run safely. Running 400m loops are less boring than you think, especially with the inclusion of a hill. I do my 20km long runs on a 1k long trail because of GI issues.

Just alternate directions and you’ll be fine.

coffee coffee coffee wrote:

Currently volunteering in another country that does not allow me to run outside due to safety reasons.

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Nothing bad will come of this. How boring it is depends on the runner. Some people would find it perfectly fine, others would find it torture.

Running with some podcasts might help if you find yourself in the latter group. Worst case scenario, you could always back off the mileage if it is too mentally grueling. At least then you'll still be doing some running.

Workouts will be the easiest, since that will break up the monotony. Easy running will be the hardest mentally.