I worked in a school where they used to invoke the word 'vocation' whenever we questioned why our hours were being ratcheted up willy nilly. Towards the end of my time there, I was working from 7.45am to 6pm every Thursday, for example, with not even a legitimate toilet break factored in, as I had to supervise playtime, lunchtime, and homework club non-stop on top of a full teaching timetable. On other days, I was required to work 7.45am to 4pm without a break, as we had to sit with the children at lunchtime and supervise them while we ate, and supervise kids who could or would not go out to play at other times, for whatever reason. I also had to coach sport, despite not being qualified to do so or knowing or caring about the rules of things like football.
I would have a quick cup of tea at 4, and then I would either attend a staff meeting until 6 (every Wednesday), or put up displays, repair school instruments, and/or prepare my classroom for the next day. On top of this I had to do all my lesson planning and marking, drive children to music exams and accompany them, drive the school minibus on trips, and take part in staff training at the weekends and during the school holidays. I was also required to babysit children when their parents couldn't be bothered to pick them up. I had a reputation for being 'difficult' as I would question all this and wonder why only certain teachers were being required to give this level of effort while others had much lighter loads.
This was all for the absolute minimum they were legally allowed to pay me, was not the terms and conditions I had thought I had signed up to, and left me and my DD absolutely exhausted. For that and other reasons I resigned. Meanwhile the head sat in his office drinking coffee, sleeping (we caught him a few times), and writing raving religious and philosophical tracts for self-publication.
If Gove thinks there is slack in the system for introducing extended schooling on this basis using existing resources, then I think he doesn't spend enough time in schools these days.