Chris, yes. I change gears to target staying within the heart rate zone. With undulating rides you still get periods of the rate dropping and also using the lowest gears, some climbs simply lift you straight into higher heart rates. I've ridden on a 42/52 for years but I've a feeling I really ought to change to a 39/53 for prcisely the point you're highlighting.

To clarify on the food, it's not so much resorting to junk as getting variety of good food whn you're late, tired, rushing, not in the frame of mind to put effort in etc. The mere mention of soup, chicken and other things wakes up parts of the memory which have gone to sleep. I really need to write down a list of alternatives and pin them inside kitchen cupboards. It's just that bit of inspiration sometimes which is required.