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just about building up and getting used to distance
^ this.
I disagree with @Arducius, although acknowledge there's more than one way to get faster.
More slow miles from extra runs at this stage will set you up better with lower risk of injury & give you a good foundation for the future. Gradually build your weekly mileage from 11 up to mid 20's (should take a few weeks so a bit late for this hm).
It sounds like a relatively long slow run on Sunday mornings is what is really missing at the mo & speedwork should only come in once you have that foundation.
Training for my second half marathon coming up in October. I want to get a more competitive time than my previous 1:44 but not sure the best way to go about it. Current training is roughly one run to work a week (11-12km) and one after work a week (7-8km). Should I try and do a couple of short, fast runs as well? Or is it just about building up and getting used to distance?