My job shipped me to Philadelphia this week, I was able to attend part of Penn Relay. Fun stuff. The 16 years old Brogan MacDougall won the 5k in front of the college women.
Sorry about the blanket statement, it was a quick response to a jest after all.
Let's be more specific. I'll only talk about static stretching.
The general idea behind static stretching being bad is that flexibility in general is bad for running economy. The way we run it's better if the muscles provide greater elastic energy return, which is when they are stiffer. Fistwell already covered that, but here are some citations:
The association between flexibility and running economy in sub-elite male distance runners.
Running economy is negatively related to sit-and-reach test performance in international-standard distance runners.
Now, let's go over comme stretching use cases.
Before a run
You're better off doing a warmup or dynamic stretching to activate the muscles. In my opinion, unless you are racing or doing a workout, you simply should warm up by starting your run slowly.
It's been clearly proven bad to static stretch before sprints. I'm pretty sure I read similar stuff regarding longer distance but I cannot find it for now.
The acute effects of static stretching on the sprint performance of collegiate men in the 60- and 100-m dash after a dynamic warm-up.
During a run
Why would you stop to stretch? Keep running.
After a run.
If someone tells you it helps clear lactic acid they're just plain wrong. Your body returns to normal lactic acid levels after an hour or so of rest.
A lot of people swear by static stretching after a run to reduce delayed onset muscle soreness. Studies show it does nothing.
Effects of stretching before and after exercising on muscle soreness and risk of injury: systematic review.
What actually helps is going for a recovery run: running super slow, just to get more blood flow to the muscles. Massages can also help but there is no clear consensus.
Now, everyone is different. If you feel like static streching after a run helps you, by all means do it. Feeling good about running is the most important thing. Unless you're an elite, the miniscule potential running economy loss incurred by stretching is not worth overthinking so much.
I had various issues with tendons on my right foot in the past. What saved my life is doing eccentric heel drops. A LOT of them, like 100x morning and evening, or until I was tired. This helped even when my Achille was not the problem.
I'm not sure if it can helps you more than your prescribed band exercices. Don't give up, not running is the worse.
Congrats man. Take it easy for a while. Take time to recover well and be careful not to come back too fast/too intense.