Long note but I hope some details and background help as I’m looking for some advice and very impressed with what I’m seeing on these forums.
My background - 45 y/o, riding about 4-5 years now, up to around 5k miles/year, tend to enjoy training more for endurance (FTP) than short power (never done a crit; like century rides), around 3.4-3.7 W/kg past couple years.
Back in mid 2016, I tore left hamstring badly enough to result in internal bleeding - likely stage 2 - couldn’t walk on it for a couple days, maybe 3-4 weeks before I could ride at all, and after a couple months working back up the power, stretching regularly, more or less back to normal on bike and got in a couple thousand miles the rest of the year. Self diagnosed/recovered as I was on HSA plan and frankly expected not much value from US health care system, and seems like pretty straightforward situation - RICE and work back up slowly. Right hamstring was strained but not damaged anywhere near same level. (aside - I don’t recommend attempting to waterski if you like cycling)
While I thought all was good, there are some long term lingering problems that I think I still need to do something about.
I found that 6 months after injury, 2000 cycling miles behind me, attempting to run on winter vacation still caused very sharp pain - ran a few km 3-4 times pushing through it, but was not pleasant. A year and a half later, following winter vacation, another maybe 4000 miles of cycling behind me, again, on vacation, ran a bit more and even sprinted a bit (short race with house mate because, why not… we’re in the Bahamas…) and it was generally a lot more tolerable but could still tell it’s not totally right. That’s the extent of my running rehab - I hate jogging 
I also had a few sessions way back around 7 months after the injury over winter where I did some leg curls on machine - it was pathetically weak despite a lot of riding that season. But going to gym was just not something I could stick with and I made excuses for not doing anything at home, like I don’t know what to do. (going to cure that here?)
The other ongoing issue which has been less and less of a problem but still hits me once in a while is that the only place I’ve ever cramped on rides is that left hamstring, and only on rides getting up near 100 miles - it just can’t keep up with the other side. Also, after some longer hard rides, if I tuck my foot under me, even sitting in chair, contracting the hamstring, it will often set it off into a painful cramp. It’s not a big deal as I have learned to mostly not do this - but this never happens with my right one.
So while I’ve learned to live with this - I have to wonder how much I’m leaving on the table having never really fully rebuilt this left hamstring to match right based on the ongoing weakness. My understanding is that hamstrings don’t play a huge role in cycling power, but since I have a stages L-only PM, anything I do to help it will fool me into thinking I’m getting twice the benefit - so the motivation will be there 
I’m going to start following core advantage stretch plan (Tom Danielson book) but if I can supplement this with at-home work to build strength, recognizing I’m also doing a low volume SS plan and starting to ride more, I’d definitely like to do something at home for this.
Interesting in anyone else that has been through something similar or good ideas on how to get this weak side strengthened back up. I’d run if it’s the #1 thing I should do to build that strength up, but I wouldn’t want to mess up cycling training too much - and what kind of running?
I have a great health plan now too, but again, I’m not sure what I’d get out of any diagnostics at this point, so haven’t really bothered - but am open to anything that I should consider in that regard.
Thanks!