The challenge is keeping it up beyond the few months when the novelty wears off, life gets in the way, and it becomes difficult.
I personally find that these kinds of things keep my interest for about about three months so I have to find ways to make progress towards goals while also changing whatever I'm doing enough to continue to hold my interesting. Examples might include changing my workout routine in a way that is different enough to hold my interest but similar enough to continue making progress. Or switch to learning a different style of guitar. Stuff like that.
Having defined goals to begin with helps a lot.
I'm no expert, but one thing that seems to help me is exploiting my natural laziness and bad habits. For example, I got in the habit of getting a nice expresso during the day from a place that's about a mile's walk away. Kind of dumb, but it built a walk into my day that I pretty rarely missed.
Along the same lines, I chose a place to live that involved a fairly long walk to the train. In principle I could have started taking a cab to the station, but I'm just to lazy to make a change like that, and indeed that worked for years (until I moved).
And that's an important point about regression. For me good habits tend to break after about 2 weeks of non-compliance. Then it often ends up being months or even years before getting back on track. At which point it's like starting over.
But the fact I got back to where I was the second time taught me you can get it all back with enough work. You still have to do it, of course, but you should only regret losing progress if you fail to get back on track when life is telling you it's time to. But don't regret the fact that life can, and will, push you off the track now and then. Embrace it, accept it, and get yourself prepared to build it back up again.
The worst thing you can do is fall off track (which will happen) and then think because of that it's not worth starting over again. Or that what you accomplished wasn't worth it because you lost it. Or that it was a one time gift, that you failed to keep. The gift is the fact that you now know you can do it, and so can do it again.
Having the recognition that it is a cycle and you have the means and the will to correct it out of the trough is the only way to really ensure life long habits. It's easier to maintain than to build, of course, but don't feel like rebuilding is totally avoidable, and that it's a failure to have to do so when life gets in the way.
Months of progress wiped just like that, and now I'm worse off than before I started running regularly.
I've never had any sort of injury like this, which means I never learned how to recover or avoid them when doing so would have been easier (when younger).
It just did. Halfway through my usual route, I started to notice a light ache around the knee. Initially I didn't think much of it, I figured it's just a little stronger than normal muscle soreness, nothing unusual for exercise. But I did lighten my pace a bit (not that I was pushing hard to begin with). About half a mile later I switched to walking because I didn't feel comfortable. I walked home OK and it wasn't terribly painful at that point, took a shower, ate, relaxed a bit and by the late evening it had gotten much worse and I couldn't really take a step with that leg.
The other knee was similar but different. Started feeling light ache when I took steps. It never got very bad but I stopped running. What was bad, and painful, really painful, was pulling on the leg. Like when you pull your pants off when changing clothes, or when helping a tight shoe off with the hand. Apart from that, it was always an annoying ache when taking steps. Rest and it goes away. Walk to the grocery store and back, and the ache is back, and so is the pain when you take pants off.
> and do you have any suggestions for avoiding it?
Not really. Checking with a physiotherapist or some experienced running coach would probably be ideal, but I guess the alternative is to just.. not push as far as I did. Give your body plenty of time to adapt, increase the load only in tiny increments over months, and immediately stop at first sign of pain? But as you say, it's hard to gauge. I didn't think I was pushing hard at all! If anything, I felt like I was in great shape to run longer and faster than I did.
If you do a search for knee injuries & strength training exercises, you'll also find that there's supposedly a lot you can and probably should do to improve muscle strength to stabilize the knee and let other parts of the body take up some load to reduce the likelihood of injury.
https://en.wikipedia.org/wiki/Iliotibial_band_syndrome
Non-specific knee ache leaving you crippled walking down stairs, and some other specific movements that cause the iliotibial band (on the outside of your knee) to rub over the bone/bursa. Super common running injury.
For me, the fix is - ice, stretching, instantly stop running if I feel a twinge, and, most importantly, ramp up mileage very very slowly (first week, 1 block, second week, 2 blocks, third week, ruin 3 blocks). Learning what it really means to "ramp up slowly" was the big challenge for me - it's always too tempting to go "hey, I used to be able to run 10mi easily, I'll just crank out an easy 5" - boom, limping for a week.
As you said, and what a nurse told me I needed stretching. That's helped against part of the ache, but for the running, I read the book born to run. They talk about using your calf muscle as a spring (sorry if this is the wrong words, I am not a native english speaker).
To do this you need to take short steps where you land your feet under you all the time so you don't put your heel in the ground. Very tough training to begin with, but it at least removed all my running problems.
Since I was a heelrunner I had to start over, since this is very tiring.
I share the same frustration with regressing back to where you started (or worse).
It seems like we all have a built-in set point for productivity. Some lucky people are just able to go, go, go, while those without such fortune are quite lazy and need pushing to do even basic tasks.
It's frustrating because you are always fighting against that regression to your set point. It takes energy, which you have less and less of as you get older. Life's hard.
Kudos to your success. Please do not get complacent. I had a similar brief transformation at the start of this year and felt on top of the world for a couple months.
Then I got Achilles tendonitis which made even walking extremely painful for about 6 weeks. My son's mom died after a long fight with cancer. That same week my current girlfriend was diagnosed with cancer. And then quarantine happened.
I let these events allow me to regress back to worse off than I was when I started. And it all felt so easy at first