Background - 46 yo, overweight, but only 6 lbs over where I used to be able to run much faster (9:00 mm vs 12:00 mm)

Have spent the last 4ish years running slower, doing trail ultras and about 1 ironman a year. Have never been fast, but my run pace is shockingly low compared to what I'm used to.

Been training consistently at low volume (by ultra and IM standards), similar to previous training volume. I CANNOT go fast. No matter how hard I try. My old normal run pace was 9-10:00 mm. Now I can barely break 12 even if attempting intervals. I feel completely stuck in 2nd gear or something. It's like my body cannot convert any energy. It's been this way for at least a year now, fairly consistent training throughout. Did an iron distance this year (very, very slowly.) Took a month off/super easy. Now attempting ramp back up and every run feels so weird. Like I should just be able to pick up the pace, but the parking brake is on.

I'm used to the swings mid season to off season, and this is totally different. This feels physiological, like there is a literal barrier preventing me from going any faster than a true slog. Definitely not over trained. Current run volume about 20 mpw, with some added fast hiking.

I don't know what to do??