Not sure what would cause this, but try different things to see what exacerbates the pain and what doesn't i.e try wetsuit and no wetsuit, pullbuoy to isolate the legs and see if your kick is to blame, try adjusting your body position - it could be your back causing the pain, perhaps your top half is too upright in the water putting stress on the back.

Here is what I think:
1. You are not old. You are not as fast as you were at 47,37 or 27 but you have lots of good years of running left.
2. Why are you now trying to fix something you have spent a lifetime doing? Instead of fixing form I recommend adding gym sessions to maintain leg strength.
3. It’s not the swimming. 4-5K of swimming per week just isn’t very much.
4. Your last sentence explains the issue.

I think the form work is bringing some muscles along the posterior chain into play that weren’t doing much before, so they get tired/sore more easily. If it’s getting worse, that’s probably wrong.

Maybe you’re straining yourself in an awkward position to achieve that form, which is causing the pain, hard to know.

Sincerely doubt it’s the swimming unless the pain only occurs after swimming.

hope you sort it out but that sounds like pain from running not pain from swimming imo.

Could be stress on the back, thought of that. I'll try and change up different variables.

Thanks. I know im not tat old but my form has definitely changed to the shuffle. not nearly as high kneed or glute mediated than in younger days. The bounce is gone! It is most noticeable when running on trails...very easy to trip as feet dont get very high off the ground.

I agree strengthening is a good idea and I have incorporated that into my training. I have spent a lifetime running but my running today is NOT the same as my running 30 yrs ago. Its lazier and more sclerotic. Trying to retain good form is a good thing.
It is not much swimming agreed. The pain comes after swimming though. not after running.

4. yes. my swimming and running have likely been coadapted. swimming changes are needed as well.

I think you are right. Reactivating those muscle is putting them sub threshold for pain as they work harder and differently. The swimming is putting them over the edge. I mentioned this in a previous post but it was your post that made me think of it...I guess that my running and swimming have coadapted over the years and the changing of running is going to require a change in swimming as well. Hopefully it will happen on its own or I will have to analyze the swim stroke and attempt to sort it out.
The pain happens after swimming and lasts for a day or two. It never happens after running.

Did you go from not swimming at all to swimming 4-5k straight once a week? Or is the 4-5k spread out over multiple times in a week?

Tim

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http://www.magnoliamasters.com
http://www.snappingtortuga.com
http://www.swimeasyspeed.com

 
I've swim regularly for years. 4-5k per week is over multiple swims. In fact is historically speaking pretty low volume.

Then it's the triathlon reflex of blaming everything on the swim when the issue is always the over-training on the bike and run where all the fatigue is being generated. And then I would look to this, "Never had it prior to running form changes. " Everything you mentioned as changes you were trying to accomplish in your run would impact your posterior chain.


Tim

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http://www.magnoliamasters.com
http://www.snappingtortuga.com
http://www.swimeasyspeed.com

Agreed. Should have phrased it differently.
Running changes causing pain after swimming would have been a better title

Reading through this I had a thought about it. If you have been swimming mostly open water, has it been a little rougher than usual?, doesn't need to be by much.
I have found in the past that if I have to arch slightly, when there are waves to sight over, I get hamstring cramps.
Also if you have been doing hamstring strengthening (you mentioned the shuffle) the same thing can happen.

Either way you may need to do some work on being able to arch without triggering your hamstrings.

My gut feel is stress on back from arching to sight in open water.