High heart rate and “struggling” makes me think this is a likely underlying cause.
Does “struggling” mean burning lungs and side stitches? If so it’s certainly this. If it’s just muscle fatigue it might also be hydration or electrolyte shortage.
Quick links on how to improve breathing while running: https://marathonhandbook.com/how-to-breathe-while-running/
Maybe it’s your fast twitch vs slow twitch muscles composition? https://blog.nasm.org/fitness/fast-twitch-vs-slow-twitch
Hard to say since you state you do other activities that would usually indicate a balanced ratio, but form of those activities can influence whether one type or the other is preferred in muscle growth… You did say sprinting short distances isn’t an issue, when you do weight training is it burst lift or slow reps?
Genetics is a big component because you most likely developed a form complimentary to your physique, meaning you do faster lifts and fewer sets because you have more type II, which then encourages growth of type II. Nothing wrong with this form or exercise approach, but it means you’re adapted to bursts of activity instead of longer duration activity. So it’d take a longer timeframe to try to retrain your muscles (if you’d even want to).
Hard to diagnose over an internet forum, a qualified trainer should be able to advise better
Edit: actually looking into it more, baseball is a sport that heavily prioritizes fast-twitch. If you trained for that in your formative growth years you likely do have an imbalance favoring type II over type I. Again: Check with a trainer over an internet stranger. But adjusting your strength training to favor typeI development could yield benefits to your distance running… That would require a multi-week period to see changes though :(