> Furthermore, three months of the year here it is too hot to walk;

Have you tried pushing through and getting used to it? I run for half an hour in Shanghai summer weather. Over 35 degrees and humid.

> six months, it is dark by the time I am freed enough from work to go walking

That sucks. Are there parks nearby or could you use a headlamp and wear high visibility clothing?

> and trying to get a worthwhile walk in before work is too time-consuming and potentially sleep-depriving.

Ten minutes of a high intensity interval teaching training routine is no fun but it can be done every morning.