You don't need a power meter to do steady state intervals. When people say to do an interval at "FTP" they're basically saying at an effort level equivalent to what you could just sustain for one hour.
Training by heart rate that same effort level is commonly referred to as lactate threshold heart rate. You can calculate that HR, by taking your average HR from a one hour effort, or 95% of a 20 minute effort, or 92% of the average of 2 8 minute efforts.
Or you can just do the steady state intervals at 8/10ths perceived effort.
As long as you're doing some harder efforts on your shorter days to get maximum benefit from your limited training time, you don't need to get overly precise about all this.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.