Nearly forgot to log the new progress.
I got the intervals slightly wrong, and did 5 + 8 + 5, with 2 minute rests instead of 3 minutes. I should hope I can manage that, having already run for longer than that a few times. The first 5 minutes, I just clocked it up to level 7 of 20, which is enough to push my heart rate up to over 140 bpm. Then in the 8 minutes, I did the same for the first 4 minutes, but put a "hill" in the way after 4 minutes. I think I might have even eventually cranked the resistance up to 11 (and I find 9 hard going), but didn't spend more than 30 seconds on the heaviest setting. Gradually as the minutes clocked up, I kept setting it down (otherwise I would've had to stop for an ambulance). The final 5 minutes also had a "hill" in it, but this one was not as steep, and just had longer periods of grunting along at level 8. The last minute was all the way down to level 6, I think.
The thing is I'm surviving it, but I had to go back these few steps to accommodate the different muscles involved. Today I didn't need to stagger out, at risk of falling down if put off balance, so it's getting better.
To begin with, I thought I was doing a bit of overkill by going all the way back from the Week 7 timings to Week 5's for this, but it turned out to be necessary. The new muscles need to build first. With the difficulty I was having in standing up when done, I even thought of going back a bit more, but today that's no longer possibly needed.
For anyone thinking of including some spinning in their training (especially as your Winter approaches), one thing I've read is that you can't treat it like cycling. In cycling, you're meant to hit quite high cadences on at least the milder slopes, so you really do "spin"; however, if you literally spin your spinning cycle, you're just putting a lot of momentum into a flywheel, which starts to help out too much, and deprives you of exercise. Because of that, you're meant to rather adjust the resistance to increase your effort, and "spin" at a slightly lower cadence. I might later find out that this is rubbish, but for now, that's what I'm doing, and why.