If I closed a hard 300 session in 39.9 what does this say about my possible 400 fitness?

52.99 not a hundredth faster

No way it's just barely under 53. Assuming it's a flying 400 (distance guys, especially those who aren't mileage-based 800 runners, never practice starts at all so I don't count that as it'll be terrible) it's easily 51-low at worst. You can't run 300 at 400 pace at the end of a workout anyway, but followed by some tempo it's probably closer to ~500-600 pace. If I'd put a # on it it's ~550m pace but that's not a distance people run at all

I've never run this exact workout, but I've done stuff like 5x mile @ tempo followed by 5x300 @ 1500 and 1x300 faster, and I'd barely be able to break 800 pace on the last 300. It'd be something like 5:00-ish miles, 43-45 300s, but I closed multiple in 39.0 to 38.5 when I could probably run 1:48 with specific training, 1:50 off mile to 5000 training.

We were doing 41 second 300s my junior year of hs in December and my buddy was splitting 51s

I never got to run the 400 off that :(

Nah to run a 49 you need to run a lot faster than 39.9 as your last effort 300 in training. More like a 36, 37 at least.

Yeah but that wasn't an effort one at the end of the session

I’m guessing 51-52, 800m 1:52, and mile 4:05

I know you didn’t ask for a mile, but If I’m accurate for the mile i’m probably accurate for the rest of it.

You aren’t far from where I’m guessing you want to be for the mile, aerobic fitness is good, you are a small gain in speed away from being someone I would bet on for sub 4:00

+1, when I first broke 50 as a middle distance athlete, running a 36.x 300m rep was a non issue. I remember doing a 6 mile tempo, 10 miles of time on feet total, ending at the track and running 36.x for a 300m, then two 23.high 200s. When talking 400s endurance is never the issue for middle distance athletes believe it or not haha, it’s all about how fast you can get those legs moving. My legs were a crippled level of dead for those strides yet the fastest 200m I could’ve ran at the time was roughly 23 flat, maybe a 22 high.

Aren't you a total 800 runner though?

I think a mile/3k guy, let alone a 3k/5k or even 5k/10k guy can maintain a lot more of their (lower) speed for 300 to 400 or 600 to 800. I just think their aerobic strength and lower amount of lactate production allows them to basically maintain the pace for longer than you'd think especially when compared to the true 400/800 runners. I once ran 1:22.0 for a time trial with others (in a down week - basically a practice race) and could run 1:51 low for the real 800. I had significantly lower speed than the 400/800 guys - probably no more than 50.5, flying - but I had a ton of strength so I could reel them in over the last ~150 or so

I also never ran faster than 39.0 for a 300 in practice, but I did occasionally run 24.x (closer to 25 than 24) feeling hard but not all out.