Me!
And from the dates and info you gave, I think we were in the same race 😀. In the interest of full disclosure, I was 251/441 and can only dream of medium pack!!
I was also deflated, as my running is also getting slower despite negligible changes to my exercise routine. I actually missed the first fixture last season, so I can't directly compare to last year, but I think I was lower down than usual.
We had a mile TT on the track tonight and I was absolutely gutted to record the worst time I've ever done. My theory is that I've put on a bit of weight (3kg), and I wasn't light before (BMI 25). I also swim with our tri club and my times there are getting faster, which for me confirms that it's the weight, as I think a heavier weight is less of a hindrance to speed in the pool.
I've tried to do a bit of strength training but haven't managed to get into a routine with it. I've tried Caroline Girvan videos, but I'm not a fan so far as she doesn't talk and I find it hard to follow by looking at the screen all the time.
I've also tried the zone 2 running thing on one run so far, and want to add a long, slow run into my routine. I currently do 3 runs a week (2 speed work on track min 5k per session, 1 social 10k with hills), and three club swims.
So to summarise my answer to your question, I don't know the answer but am trying losing weight and zone 2 running as well as strength training.
Happy to share a "get-faster" journey with you!