Thanks for starting this thread.
I'm particularly interested in the topic, since as recently as four months ago I was quite sure that I would never run again. My back problems simply made it impossible. So for the past year--until three months ago--I had converted into a cyclist, averaging 70-110 miles a week.
But my back relented, and so, beginning with a tentative mile or two at 11:45 pace and a high HR, I began to run again. At age 63. Last week I notched 39 miles with a 10 mile long run--the longest I'd run in 18 months.
Here are my observations, in no particular order:
1) Learn how to give up. This is rule #1. Just learn how to quit. How to cut it short. How to pack it in. Every day is an experiment--a glorious experiment, a miserable experiment, but always an experiment that has something to teach you.
2) If cutting back on your running miles and swapping in some other activity is what you feel you need to do, do it. YOU are the final arbiter of what you need. I've discovered that I'm able to run seven days a week, more or less as I used to after my year and a half of giving up and becoming a cyclist. I run 3-4 miles on M, T, Th, and Sa, 7-8 on W/F, and whatever I can do for a LR on Sunday. 6, 8, 9, now 10. I'd like to push it out to 12. Once I've done that, of course I'll be stupid and say, "Fourteen miles, one last time."
3) Your paces--easy/recovery paces, high aerobic paces, threshold paces--will slow. Allow that. Listen to your body and trust what it tells you.
4) Listen to your body and trust what it tells you. Don't judge it. Listen to it.
5) Have I mentioned that you should listen to your body and follow its dictates?
6) I've heard many people say, "If you need an extra rest day, take it." I didn't like that wisdom, but it turns out that if you take an extra rest day sometimes, your workout on the next hard-ish day will be better--duh--and what you do might even inspire you by setting a new standard. "Wow! I can do THAT? I'm still alive and kicking." That happened to me on Christmas day, of all days, when I had the best run of the year, which is to say the best run of a two-month-long comeback. I finished a 7 mile run with an 8:25 mile! I was hammering. Not dead yet.
7) Learn how to reframe everything. Because it's all teaching. So: you feel incredibly heavy and lethargic, as I did two weeks ago. I'd planned to run 8 miles; after a mile or two, I started the mental dial-back. I ended up running 6 sucky miles. The run became about my determination simply to get through the run--and to benefit from the mental training that that unexpectedly bad run offered me. This morning I ran four miles at just under 11:00 pace, an easy/recovery run, and the whole run was about celebrating the fact that my HR was fairly low relative to other similar runs. Learning how to take those sorts of pleasures: that's where over-60 running asks you to go.
8) The long game. The sooner you let go of what you used to do and learn how to take pleasure in the incremental improvements that are still possible between where you are right now and where you were a few weeks back, the better off you'll be. It's all mental, as you know. It's all about how you frame things. I shouldn't be running AT ALL. Yet somehow I am. So every run is a gift. And that is a blessing.