Will I get faster?

So I've been running since August, I aim for 3 runs a week but sometimes it's 2 because I'm a nurse and do shifts/have a 3 year old.

I was very slow at the start, only managing to do 3km in 30 mins. I can now do 4km in 33 mins. Is this a reasonable improvement in a few months?

I'm getting frustrated that I'm not getting better quicker but I think that's me being impatient.

Is it better to go slower and do some longer runs for endurance maybe? I can run faster than 8min/km but it's a sprint and I'd have to stop more frequently.