Without making a "blanket" statement for everyone, I will only speak for myself. Being run fit takes me to a whole new level of fitness on the bike. The reverse isn't true for me, however, I find run fitness transfers quite well to the bike. As long as you are biking regularly so your leg muscles can support your aerobic system, you'll be fine.

I've found running crosses over hugely to cycling as a typical AGer (not elite/pro). Particularly for aerobic endurance - the leg endurance and cardio endurance you gain in running directly transfer over for aerobic (lower) level efforts on the bike that may have you going long.

The x-over decreases the stronger the cyclist you get, particularly the stronger you are a cyclist relative to your running as your legs get more specificity.

The x-over from cycling to running is a lot less, since the pounding on the legs in running will dramatically limit you even if you have a great cycling engine.

If I had limited time and could do predominantly run OR bike, it would be run hands down for triathlon racing.

I'll also note that being as good a triathlete as you can does NOT mean training S-B-R to be the best you can be in each sport - it really is S+B+R on race day, all in one and a good training program will reflect that. In particularly, expending a ton of effort to push to your near-limits in one particular sport will usually end up causing you to sacrifice too much in another one on race day in a triathlon to make it worth it. In triathlon, even a 90%/10% run:bike training ratio will dramatically outperform a 100/0% run:bike training ratio, and a 60/40 will crush both of those for most distances.

For the OP though: if your main "A" goal is to ride the fastest weekend event you can ride, pure cycling (no running) will outperform sacrificing some cycle time in lieu of running. (Running will not outperform cycling for training for a pure cycling event.) But if you are still interested in maintaining your running ability for a future tri,a smattering of running will minimally affect your cycling and still hold up a lot of run ability (wayyy more than zero running).

im not the strongest anything really, but i feel im a better runner than cyclist. my aerobic engine can hit max while running while my leg strength is the limiting factor on the bike. im not an expert but ive kinda gleaned that having a great running engine does translate over to the bike in a regard.

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80/20 Endurance Ambassador

Here's a helpful video on this topic.

https://www.youtube.com/watch?v=OSKNLQxZyT0

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Dimond Bikes Superfan

This part is pretty much my experience as well.

Lightheir's comments are pretty similar to my experience here. I'm mostly a runner but in the past few years have gotten more into cycling and multisport racing. Training time has averaged:
2018: ~6.5 hours/week running, 0.5 hours/week cycling
2019: ~5.75 hours/week running, 2.5 hours/week cycling
2020: ~6 hours/week running, 3 hours/week cycling

Getting into cycling more heavily in 2019 I was immediately pretty decent, and strong at climbing. Just got a powermeter recently and tested my FTP at just about spot-on 4 W/kg. I don't think I'd be anywhere near that on 3 hours/week of cycling with no running! I think running hills, and doing long runs on hilly terrain, are especially likely to transfer well to cycling endurance since the muscle firing patterns when running uphill are quite close to those when cycling. I typically run a lot of rolling and hilly terrain.

The only place where I disagree with Lightheir is that I think cycling actually transfers pretty strongly back to my running. When I up my bike mileage or time, even if I decrease running mileage some to compensate, my running seems to stay just as strong or even get faster in some cases. I haven't had enough controlled experience doing this to understand why, it seems to fly in the face of conventional wisdom about training specificity, and maybe it's some sort of peculiar result of my biomechanics or a result of unloading run-specific fatigue. But I suspect it might be a real effect that relates to the quantity of high-end aerobic work being larger when I'm biking more than running more. You can simply do more high-end aerobic work without high risk of injury on the bike than you can while running -- you'd almost never do a 2x20' tempo run, but 2x20' is the norm for threshold work on the bike. I also notice that when I do fast running off cycling-heavy blocks, I run well but my calves are the weak point, and not as acclimated to the eccentric stress as when I'm running a lot.