The cross-adaptation is almost complete, now, so it looks like it might take say a month to completely transfer what you can from one discipline to another, and say two weeks to get out of the phase where you're discovering muscles you didn't know you had.
There was enough in the legs this time to start "spin 1" (10 minutes) at level 7, and then put a hill in midway that went up to level 15 (very briefly, and with poor form). Most of the last part of spin 1 was spent "flattening off the top of the hill". It was quite a hard last 2 minutes, even coming back right down to level 6.
This time I took a 2:30 rest, instead of 2:00 (just testing how much of the "solid run fitness" is still intact), and then I did a similar second 10 minutes. This time after about a minute, I pushed it up to level 8, just to be falling down to a "run" if I hit collapso at some point. And then the hill in the middle might have gone to level 16 very briefly, too, but needed to be flattened out much more quickly than did the hill of the first 10 minutes. When I got to the end, I dropped to level 7 but fast for the last minute, and then tried to keep going for as long as possible afterwards. Part of the "keeping going" was at level 6, and slow, but then at some point I went and racked it right up to level 20 (max resistance) for a very painful short while. I fiddled around at the end, delaying slowing down, and don't remember many details.
So the discoveries that might be of some use to those thinking of mixing in some spinning with the programme? The nice thing about spinning is that you create your own hills, I suppose. And if they get too steep, instead of dropping to a "walk", you can just reduce the "slope". I would say that I now prefer spinning to treadmills at least. The adjustments you can make are more direct (eg you can change cadence).
The real measure of the effort put in is not actually the resistance level or the cadence, either. It's your power output in Watts (which the machine tells you, and which is nice to see). At times today I went over 200 Watts, and "walking effort" looks to be about 100 Watts plus or minus.
Finally, it's nice to have heart rate feedback to correlate to perceived effort (they don't seem to correlate all that well for me, though). Today I occasionally bumped a rate of 173 bpm, which is OK, but not good to sustain too long for someone in my state of fitness, I think.
There's life after running. There are alternatives to running when running is not possible. Nice to know.