I think of it like which would make you go faster - a bus or a ferrari

If you get a better bike, yes you might go faster, and be able to go for longer distances, but it won't do anything for your fitness. I think that's the key point. You need to keep up your training along with it.

I'm wanting a new lighter bike because my area is very hilly, and I don't trust myself to be able to get up some of them on a 14kg mountain bike orientated hybrid.