Why is running still so hard?

Running is always hard IME but it sounds like you're running too fast, if you're unable to hold a conversation.

I've never enjoyed running and much prefer cycling but am currently doing the C25k again after a long layoff from running.

Not everyone responds in the same way with HIIT. I seem to respond best when using a stationary bike and doing this workout:

It really opens up my lungs and does wonders for my pace.

IMO speed is irrelevant and never embarrassing. Just do what's comfortable for you.

I found running was much more comfortable after I’d strengthened my legs through weight training. It didn’t seem as painful. I thought they’d strengthen through the running but apparently not. In terms of speed, I am slightly faster but I’m not really one for measuring and recording speeds etc

I’m very slow. It takes me 40minutes to run 5km but I can talk most of the way through unless I’m on a hill. I can’t run on my own though. I run with a club so I always have someone to keep me company, I wouldn’t keep going if I didn’t have someone with me.

I’m not too far off what you describe. I’ve done c25k all the way through four times now, the final three times alongside also running 5ks a couple a week and trying to use the programme to get faster. Eg, jogging and sprinting instead of walking and jogging. I’ve also done the 5k to 10k programme. I’m still very slow. I still can’t hold a conversation when running. That’s still a massive improvement on where I was. I mean in that I can not exercise for several weeks as recently when I hurt my ankle, and then when I go out, I do do a 5k without much actual difficulty - it’s just I’m slow, and it’s getting embarrassing!

I think the best improvement I had is when doing the 5to 10 k programme so I’d maybe look at that OP