I've kind of graduated from C25K and have been using mapmyrun. It annoys me that even though it is set to km rather than miles it announces the splits every mile. So it'll say e.g. "You have run 1.6km at a pace of x"
I'd like one that I can set a pace and it'll tell me if I'm going too slow. Which I think mapmyrun does if you upgrade?
No, you don't need special trainers. I tried it but my (rather elderly) phone didn't like it. I also tried Endomondo but the free version kept sending me to a porn site every time I tried to open the app .
I've settled for RunKeeper, the free version does everything I need and there's no unwanted porn. I'm not sure about pace alarms, I don't use one. You might need to upgrade to do that.
RunDouble has a pace alarm but you can't set your own distance, you have to choose from a list.
I've got an Android, not an iPhone so I don't know if the features would be different on an iPhone version.
It has all the usual stuff plus you can set up your own activities such as intervals and customise the pace then it will tell you quite frequently to increase or decrease speed or that you're running at your pace target.....
It also has pre-programmed training plans for 5k, 10k, half snd marathon and can be used for other sports too
And .... If you stop at traffic lights, it detects time stopped and adjusts average pace etc too
I have had the apps once early days... I think to get all the features now its a premium version but it is good